Delete empty folder /System BEFORE reassigning _systemLibrariesFolder

Otherwise we will try to delete the wrong folder.
This bug would always throw an error on Linux and Windows. The installer
would then stop having completed every step except deleting the empty
System folder.

Utilities/WOInstall/Sources/er/woinstaller/WebObjectsInstallation.java

@@ -35,6 +35,15 @@ public File getSystemLibrariesFolder() {
     return _systemLibrariesFolder;
   }
 
+  /**
+   * This method moves the folders which were designed for macOS
+   * to the locations required for linux or windows
+   * If the destination folder is /opt then
+   * /opt/Library becomes /opt/Local/Library
+   * /opt/System/Library becomes /opt/Library
+   * /opt/System is then deleted
+   * @throws IOException if folders already exist or can't move folders or can't delete empty folder /opt/System
+   */
   public void renameFolders() throws IOException {
     File localFolder = new File(_destinationFolder, "Local");
     if (localFolder.exists()) {
@@ -57,11 +66,12 @@ public void renameFolders() throws IOException {
     if (!_systemLibrariesFolder.renameTo(libraryFolder)) {
       throw new IOException("Failed to move '" + _systemLibrariesFolder + "' to '" + libraryFolder + "'.");
     }
-    _systemLibrariesFolder = libraryFolder;
-
+    // Delete empty folder /opt/System BEFORE reassigning _systemLibrariesFolder otherwise we will lose reference to it
     if (!_systemLibrariesFolder.getParentFile().delete()) {
       throw new IOException("Failed to delete '" + _systemLibrariesFolder.getParentFile() + ".");
     }
+    _systemLibrariesFolder = libraryFolder;
+
     System.out.println("Done");
   }
 }