diff --git a/src/main/kotlin/me/elsiff/morefish/command/MainCommand.kt b/src/main/kotlin/me/elsiff/morefish/command/MainCommand.kt
index 2fafd1e7..7090ff52 100644
--- a/src/main/kotlin/me/elsiff/morefish/command/MainCommand.kt
+++ b/src/main/kotlin/me/elsiff/morefish/command/MainCommand.kt
@@ -148,7 +148,7 @@ class MainCommand(
                 return
             }
 
-            val target = sender.server.getPlayerExact(args[0]) ?: null
+            val target = sender.server.getPlayerExact(args[0])
             if (target == null) {
                 val msg = Lang.format("player-not-found").replace("%s" to args[0]).output()
                 sender.sendMessage(msg)
diff --git a/src/main/kotlin/me/elsiff/morefish/configuration/ConfigurationValueAccessor.kt b/src/main/kotlin/me/elsiff/morefish/configuration/ConfigurationValueAccessor.kt
index 3541558c..3c20ce78 100644
--- a/src/main/kotlin/me/elsiff/morefish/configuration/ConfigurationValueAccessor.kt
+++ b/src/main/kotlin/me/elsiff/morefish/configuration/ConfigurationValueAccessor.kt
@@ -63,13 +63,14 @@ abstract class ConfigurationValueAccessor {
 
     private inline fun <reified T> findValue(
         path: String,
-        getter: (String) -> T,
+        getter: (String) -> T?,
         typeChecker: (String) -> Boolean,
         default: T?
     ): T {
         return if (currentSection.contains(path)) {
             require(typeChecker(path)) { "Value of '$path' in configuration is not a ${T::class.simpleName}" }
-            getter(path)
+            // typeChecker also checks nullability
+            getter(path)!!
         } else {
             require(default != null) { "Path '$path' must exist or have a default value" }
             default
diff --git a/src/main/kotlin/me/elsiff/morefish/configuration/loader/EnchantmentMapLoader.kt b/src/main/kotlin/me/elsiff/morefish/configuration/loader/EnchantmentMapLoader.kt
index 9bd413f0..5907fb68 100644
--- a/src/main/kotlin/me/elsiff/morefish/configuration/loader/EnchantmentMapLoader.kt
+++ b/src/main/kotlin/me/elsiff/morefish/configuration/loader/EnchantmentMapLoader.kt
@@ -1,6 +1,7 @@
 package me.elsiff.morefish.configuration.loader
 
 import me.elsiff.morefish.configuration.ConfigurationValueAccessor
+import me.elsiff.morefish.util.NamespacedKeyUtils
 import org.bukkit.NamespacedKey
 import org.bukkit.enchantments.Enchantment
 
@@ -12,9 +13,9 @@ class EnchantmentMapLoader : CustomLoader<Map<Enchantment, Int>> {
         return if (section.contains(path)) {
             section.strings(path).map {
                 val tokens = it.split(DELIMITER)
-                val enchantment = Enchantment.getByKey(NamespacedKey.minecraft(tokens[0]))
+                val enchantment = NamespacedKeyUtils.enchantment(tokens[0])
                 val level = tokens[1].toInt()
-                Pair<Enchantment, Int>(enchantment, level)
+                Pair(enchantment, level)
             }.toMap()
         } else {
             emptyMap()
diff --git a/src/main/kotlin/me/elsiff/morefish/configuration/loader/FishConditionSetLoader.kt b/src/main/kotlin/me/elsiff/morefish/configuration/loader/FishConditionSetLoader.kt
index 5a06073b..362b82a2 100644
--- a/src/main/kotlin/me/elsiff/morefish/configuration/loader/FishConditionSetLoader.kt
+++ b/src/main/kotlin/me/elsiff/morefish/configuration/loader/FishConditionSetLoader.kt
@@ -10,6 +10,7 @@ import org.apache.commons.lang.math.DoubleRange
 import org.bukkit.NamespacedKey
 import org.bukkit.block.Biome
 import org.bukkit.enchantments.Enchantment
+import java.lang.IllegalArgumentException
 
 /**
  * Created by elsiff on 2019-01-09.
@@ -50,7 +51,7 @@ class FishConditionSetLoader : CustomLoader<Set<FishCondition>> {
                 BiomeCondition(args.map { Biome.valueOf(it.toUpperCase()) })
             "enchantment" ->
                 EnchantmentCondition(
-                    Enchantment.getByKey(NamespacedKey.minecraft(args[0])),
+                    NamespacedKeyUtils.enchantment(args[0]),
                     args[1].toInt()
                 )
             "level" ->
diff --git a/src/main/kotlin/me/elsiff/morefish/dao/yaml/YamlRecordDao.kt b/src/main/kotlin/me/elsiff/morefish/dao/yaml/YamlRecordDao.kt
index c3be1b7c..eb195cec 100644
--- a/src/main/kotlin/me/elsiff/morefish/dao/yaml/YamlRecordDao.kt
+++ b/src/main/kotlin/me/elsiff/morefish/dao/yaml/YamlRecordDao.kt
@@ -40,9 +40,9 @@ class YamlRecordDao(
 
     override fun update(record: Record) {
         val id = record.fisher.uniqueId.toString()
-        require(yaml.contains(id)) { "Record must exist in the ranking" }
-
-        setRecord(yaml.getConfigurationSection(id), record)
+        require(yaml.isConfigurationSection(id)) { "Record must exist in the ranking" }
+        // isConfigurationSection checks nullability.
+        setRecord(yaml.getConfigurationSection(id)!!, record)
         yaml.save(file)
     }
 
@@ -65,13 +65,13 @@ class YamlRecordDao(
 
     override fun all(): List<Record> {
         val records = mutableListOf<Record>()
-        for (section in yaml.getKeys(false).map(yaml::getConfigurationSection)) {
-            val id = UUID.fromString(section.name)
+        yaml.getKeys(false).filter(yaml::isConfigurationSection).map(yaml::getConfigurationSection).forEach { section ->
+            val id = UUID.fromString(section!!.name)
             val player = plugin.server.getOfflinePlayer(id)
 
             val fishTypeName = section.getString("fish-type")
             val fishType = fishTypeTable.types.find { it.name == fishTypeName }
-                ?: throw IllegalStateException("Fish type doesn't exist for '$fishTypeName'")
+                    ?: throw IllegalStateException("Fish type doesn't exist for '$fishTypeName'")
             val fishLength = section.getDouble("fish-length")
 
             val fish = Fish(fishType, fishLength)
diff --git a/src/main/kotlin/me/elsiff/morefish/fishing/catchhandler/AbstractBroadcaster.kt b/src/main/kotlin/me/elsiff/morefish/fishing/catchhandler/AbstractBroadcaster.kt
index 8d42d8d8..50a8a7a9 100644
--- a/src/main/kotlin/me/elsiff/morefish/fishing/catchhandler/AbstractBroadcaster.kt
+++ b/src/main/kotlin/me/elsiff/morefish/fishing/catchhandler/AbstractBroadcaster.kt
@@ -23,7 +23,7 @@ abstract class AbstractBroadcaster : CatchHandler {
             val receivers = fish.type.catchAnnouncement.receiversOf(catcher).toMutableList()
 
             if (Config.standard.boolean("messages.only-announce-fishing-rod")) {
-                receivers.removeIf { it.inventory.itemInMainHand?.type != Material.FISHING_ROD }
+                receivers.removeIf { it.inventory.itemInMainHand.type != Material.FISHING_ROD }
             }
 
             val msg = catchMessageFormat.replace(
diff --git a/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionHost.kt b/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionHost.kt
index 5b5c1071..ee7c7370 100644
--- a/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionHost.kt
+++ b/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionHost.kt
@@ -125,7 +125,7 @@ class FishingCompetitionHost(
         return mapOf(
             "%ordinal%" to NumberUtils.ordinalOf(number),
             "%number%" to number.toString(),
-            "%player%" to record.fisher.name,
+            "%player%" to (record.fisher.name ?: "null"),
             "%length%" to record.fish.length.toString(),
             "%fish%" to record.fish.type.name
         )
diff --git a/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionTimerBarHandler.kt b/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionTimerBarHandler.kt
index f7f3e0ef..0cb92f7f 100644
--- a/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionTimerBarHandler.kt
+++ b/src/main/kotlin/me/elsiff/morefish/fishing/competition/FishingCompetitionTimerBarHandler.kt
@@ -49,12 +49,12 @@ class FishingCompetitionTimerBarHandler(
         barUpdatingTask = null
 
         timerBar!!.apply {
-            title = timerBarTitle(0)
+            setTitle(timerBarTitle(0))
             progress = 0.0
             removeAll()
         }
 
-        HandlerList.unregisterAll(barDisplayer)
+        barDisplayer?.let { HandlerList.unregisterAll(it) }
         barDisplayer = null
 
         plugin.server.removeBossBar(timerBarKey)
@@ -74,7 +74,7 @@ class FishingCompetitionTimerBarHandler(
         override fun run() {
             remainingSeconds--
             timerBar!!.run {
-                title = timerBarTitle(remainingSeconds)
+                setTitle(timerBarTitle(remainingSeconds))
                 progress = remainingSeconds.toDouble() / duration
             }
         }
diff --git a/src/main/kotlin/me/elsiff/morefish/fishing/competition/Prize.kt b/src/main/kotlin/me/elsiff/morefish/fishing/competition/Prize.kt
index fb79cc29..794eb7dd 100644
--- a/src/main/kotlin/me/elsiff/morefish/fishing/competition/Prize.kt
+++ b/src/main/kotlin/me/elsiff/morefish/fishing/competition/Prize.kt
@@ -17,8 +17,10 @@ class Prize(
         }
 
         val server = plugin.server
-        for (command in commands) {
-            server.dispatchCommand(server.consoleSender, command.replace("@p", player.name))
+        player.name?.let {
+            for (command in commands) {
+                server.dispatchCommand(server.consoleSender, command.replace("@p", it))
+            }
         }
     }
 }
\ No newline at end of file
diff --git a/src/main/kotlin/me/elsiff/morefish/fishing/condition/EnchantmentCondition.kt b/src/main/kotlin/me/elsiff/morefish/fishing/condition/EnchantmentCondition.kt
index 8815d0a7..7aca5b24 100644
--- a/src/main/kotlin/me/elsiff/morefish/fishing/condition/EnchantmentCondition.kt
+++ b/src/main/kotlin/me/elsiff/morefish/fishing/condition/EnchantmentCondition.kt
@@ -14,7 +14,7 @@ class EnchantmentCondition(
         fisher: Player,
         competition: FishingCompetition
     ): Boolean {
-        val fishingRod = fisher.inventory.itemInMainHand ?: return false
+        val fishingRod = fisher.inventory.itemInMainHand
         return fishingRod.containsEnchantment(enchantment) && fishingRod.getEnchantmentLevel(enchantment) >= minLevel
     }
 }
\ No newline at end of file
diff --git a/src/main/kotlin/me/elsiff/morefish/fishing/condition/PotionEffectCondition.kt b/src/main/kotlin/me/elsiff/morefish/fishing/condition/PotionEffectCondition.kt
index ff58e28f..6648e275 100644
--- a/src/main/kotlin/me/elsiff/morefish/fishing/condition/PotionEffectCondition.kt
+++ b/src/main/kotlin/me/elsiff/morefish/fishing/condition/PotionEffectCondition.kt
@@ -10,6 +10,6 @@ class PotionEffectCondition(
     private val minAmplifier: Int
 ) : FishCondition {
     override fun check(caught: Item, fisher: Player, competition: FishingCompetition): Boolean {
-        return fisher.hasPotionEffect(effectType) && fisher.getPotionEffect(effectType).amplifier >= minAmplifier
+        return fisher.hasPotionEffect(effectType) && fisher.getPotionEffect(effectType)!!.amplifier >= minAmplifier
     }
 }
\ No newline at end of file
diff --git a/src/main/kotlin/me/elsiff/morefish/hooker/VaultHooker.kt b/src/main/kotlin/me/elsiff/morefish/hooker/VaultHooker.kt
index 4e1ea7ff..96276e05 100644
--- a/src/main/kotlin/me/elsiff/morefish/hooker/VaultHooker.kt
+++ b/src/main/kotlin/me/elsiff/morefish/hooker/VaultHooker.kt
@@ -14,7 +14,7 @@ class VaultHooker : PluginHooker {
     override fun hook(plugin: MoreFish) {
         PluginHooker.checkEnabled(this, plugin.server.pluginManager)
 
-        val registration = plugin.server.servicesManager.getRegistration(Economy::class.java) ?: null
+        val registration = plugin.server.servicesManager.getRegistration(Economy::class.java)
         if (registration != null) {
             economy = registration.provider
         }
diff --git a/src/main/kotlin/me/elsiff/morefish/item/FishItemStackConverter.kt b/src/main/kotlin/me/elsiff/morefish/item/FishItemStackConverter.kt
index 3e33e4c5..a7a2fb0a 100644
--- a/src/main/kotlin/me/elsiff/morefish/item/FishItemStackConverter.kt
+++ b/src/main/kotlin/me/elsiff/morefish/item/FishItemStackConverter.kt
@@ -31,11 +31,13 @@ class FishItemStackConverter(
     }
 
     fun isFish(itemStack: ItemStack): Boolean {
-        return fishReader.canRead(itemStack.itemMeta)
+        val itemMeta = itemStack.itemMeta ?: return false
+        return fishReader.canRead(itemMeta)
     }
 
     fun fish(itemStack: ItemStack): Fish {
-        return fishReader.read(itemStack.itemMeta)
+        require(isFish(itemStack)) { "The itemStack is not fish" }
+        return fishReader.read(itemStack.itemMeta!!)
     }
 
     fun createItemStack(fish: Fish, catcher: Player): ItemStack {
@@ -43,7 +45,7 @@ class FishItemStackConverter(
         if (!fish.type.hasNotFishItemFormat) {
             val replacement = getFormatReplacementMap(fish, catcher)
             itemStack.edit<ItemMeta> {
-                displayName = formatConfig.format("display-name").replace(replacement).output(catcher)
+                setDisplayName(formatConfig.format("display-name").replace(replacement).output(catcher))
                 lore = formatConfig.formats("lore").replace(replacement).output(catcher)
                 fishWriter.write(this, fish)
             }
diff --git a/src/main/kotlin/me/elsiff/morefish/item/FishItemTagReader.kt b/src/main/kotlin/me/elsiff/morefish/item/FishItemTagReader.kt
index 9e02c6d4..dd3a3e84 100644
--- a/src/main/kotlin/me/elsiff/morefish/item/FishItemTagReader.kt
+++ b/src/main/kotlin/me/elsiff/morefish/item/FishItemTagReader.kt
@@ -28,7 +28,7 @@ class FishItemTagReader(
             val typeName = tags.getCustomTag(fishTypeKey, ItemTagType.STRING)
             val type = fishTypeTable.types.find { it.name == typeName }
                 ?: throw IllegalStateException("Fish type doesn't exist")
-            val length = tags.getCustomTag(fishLengthKey, ItemTagType.DOUBLE)
+            val length = tags.getCustomTag(fishLengthKey, ItemTagType.DOUBLE) ?: 0.0
             Fish(type, length)
         }
     }
diff --git a/src/main/kotlin/me/elsiff/morefish/shop/FishShopGui.kt b/src/main/kotlin/me/elsiff/morefish/shop/FishShopGui.kt
index 87fe79ea..4e8822ec 100644
--- a/src/main/kotlin/me/elsiff/morefish/shop/FishShopGui.kt
+++ b/src/main/kotlin/me/elsiff/morefish/shop/FishShopGui.kt
@@ -40,7 +40,7 @@ class FishShopGui(
 
     init {
         val bottomBarIcon = ItemStack(Material.LIGHT_BLUE_STAINED_GLASS_PANE)
-        bottomBarIcon.edit<ItemMeta> { displayName = " " }
+        bottomBarIcon.edit<ItemMeta> { setDisplayName(" ") }
         for (slot in bottomBarSlots) {
             inventory.setItem(slot, bottomBarIcon)
         }
@@ -94,16 +94,16 @@ class FishShopGui(
 
     private fun allFishItemStacks(): List<ItemStack> {
         return fishSlots
-            .mapNotNull { slot -> inventory.getItem(slot) ?: null }
+            .mapNotNull { slot -> inventory.getItem(slot) }
             .filter { itemStack -> converter.isFish(itemStack) }
     }
 
     private fun updatePriceIcon(price: Double = totalPrice) {
         val emeraldIcon = ItemStack(Material.EMERALD)
         emeraldIcon.edit<ItemMeta> {
-            displayName = Lang.format("shop-emerald-icon-name")
+            setDisplayName(Lang.format("shop-emerald-icon-name")
                 .replace("%price%" to price.toString())
-                .output()
+                .output())
         }
         inventory.setItem(priceIconSlot, emeraldIcon)
     }
diff --git a/src/main/kotlin/me/elsiff/morefish/shop/FishShopSignListener.kt b/src/main/kotlin/me/elsiff/morefish/shop/FishShopSignListener.kt
index 4ac01bc4..d1c299ec 100644
--- a/src/main/kotlin/me/elsiff/morefish/shop/FishShopSignListener.kt
+++ b/src/main/kotlin/me/elsiff/morefish/shop/FishShopSignListener.kt
@@ -35,10 +35,10 @@ class FishShopSignListener(
 
     @EventHandler
     fun onPlayerInteract(event: PlayerInteractEvent) {
-        if (event.action == Action.RIGHT_CLICK_BLOCK &&
-            event.clickedBlock.state is Sign
-        ) {
-            val sign = event.clickedBlock.state as Sign
+        if (event.action != Action.RIGHT_CLICK_BLOCK) return
+        val clickedBlock = event.clickedBlock ?: return
+        if (clickedBlock.state is Sign) {
+            val sign = clickedBlock.state as Sign
             if (sign.lines[0] == shopSignTitle) {
                 if (Config.standard.boolean("fish-shop.enable")) {
                     fishShop.openGuiTo(event.player)
diff --git a/src/main/kotlin/me/elsiff/morefish/util/InventoryUtils.kt b/src/main/kotlin/me/elsiff/morefish/util/InventoryUtils.kt
index d5e9191f..95de1785 100644
--- a/src/main/kotlin/me/elsiff/morefish/util/InventoryUtils.kt
+++ b/src/main/kotlin/me/elsiff/morefish/util/InventoryUtils.kt
@@ -12,7 +12,7 @@ object InventoryUtils {
     fun emptyStack(): ItemStack = ItemStack(Material.AIR)
 
     fun deliverTo(inventory: Inventory, delivery: ItemStack, acceptableSlots: List<Int> = inventory.slots()) {
-        val contents = acceptableSlots.mapNotNull { slot -> inventory.getItem(slot) ?: null }
+        val contents = acceptableSlots.mapNotNull { slot -> inventory.getItem(slot) }
         for (invItem in contents.filter { it.isSimilar(delivery) }) {
             val givingAmount = min(delivery.amount, invItem.maxStackSize - invItem.amount)
             invItem.amount = invItem.amount + givingAmount
diff --git a/src/main/kotlin/me/elsiff/morefish/util/NamespacedKeyUtils.kt b/src/main/kotlin/me/elsiff/morefish/util/NamespacedKeyUtils.kt
index 8ce12a0a..b056a8ca 100644
--- a/src/main/kotlin/me/elsiff/morefish/util/NamespacedKeyUtils.kt
+++ b/src/main/kotlin/me/elsiff/morefish/util/NamespacedKeyUtils.kt
@@ -2,6 +2,7 @@ package me.elsiff.morefish.util
 
 import org.bukkit.Material
 import org.bukkit.NamespacedKey
+import org.bukkit.enchantments.Enchantment
 import org.bukkit.potion.PotionEffectType
 
 /**
@@ -27,4 +28,11 @@ object NamespacedKeyUtils {
         return PotionEffectType.getByName(namespacedKey.key)
             ?: throw IllegalStateException("There's no potion effect type whose id is '$namespacedKey'")
     }
-}
\ No newline at end of file
+
+    fun enchantment(id: String): Enchantment = enchantment(fromMinecraft(id))
+
+    fun enchantment(namespacedKey: NamespacedKey): Enchantment {
+        return Enchantment.getByKey(namespacedKey)
+                ?: throw IllegalStateException("There's no enchantment whose id is '$namespacedKey'")
+    }
+}