Fixes strangerstudios#115: Resolve undefined 'link_class' warning for…

… group join button.

includes/restrictions.php

@@ -76,10 +76,15 @@ function pmpro_bp_bp_user_can_create_groups( $can_create, $restricted ) {
  * Hide the Join Group button if joining groups is restricted
  */
 function pmpro_bp_disable_group_buttons( $button_args, $group ) {			
-	if( ! isset( $button_args['id'] ) || ( $button_args['id'] === 'join_group' || $button_args['id'] === 'request_membership' || $button_args['id'] === 'group_membership' ) && !pmpro_bp_user_can_join_group( $group->id ) ) {
+	if( !pmpro_bp_user_can_join_group( $group->id ) 
+		&& (! isset( $button_args['id'] ) 
+		|| ( $button_args['id'] === 'join_group' 
+		|| $button_args['id'] === 'request_membership' 
+		|| $button_args['id'] === 'group_membership' ) ) ) {
+
 		global $pmpro_pages;
 		$button_args['link_href'] = get_permalink($pmpro_pages['pmprobp_restricted']);
-		$button_args['link_class'] = str_replace( 'join-group', '', $button_args['link_class'] );
+		$button_args['link_class'] = str_replace( 'join-group', '', $button_args['link_class'] ?? '' );
 		$button_args['button_element'] = 'a';
 	}