Starting JavaFX from Main method of class which doesn’t extend Application

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In addition to what Nejinx said, you must not directly call your start(), always call launch(), because it sets up the JavaFX environment, including creation of stage and calls start() passing the stage as an parameter to it.

The docs has a note specially stating this

NOTE: This method is called on the JavaFX Application Thread

The launch() can be called from any class, taking into consideration if the class is directly not extending javafx.application.Application, then you must pass the class extending it as an argument to the launch method.

For example, consider you have a class JavaFXMain which extends Application

class JavaFXMain extends Application {...}

You can use any other class, to start the JavaFX Application.

class Main {
   ...
   public void someMethod() {
      ...
      JavaFXMain.launch(JavaFXMain.class); // Launch the JavaFX application
      ...
   }
}

In your case, you can try something like this inside the main method of MainApp:

// You may remove args if you don't intend to pass any arguments
MainUIController.launch(MainUIController.class, args)

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