Create a new "JavaFx FXML Application" project from Netbeans, using the defaults.
Open Sample.fxml with SB, drop a new Button.
From the Code section set onAction to "newAction" method (you unfortunately forgot the heading '#').
Add the method in the SampleController.java:
    private void newAction(ActionEvent event) {
        System.out.println("You clicked new Button!");
    }

Start the project.
You get an incomprehensible message : "Page language not specified." enclosed in exceptions (see below).

This kind of issue can be very difficult to solve for a user.


Page language not specified.
file:/C:/Users/jcambon/Documents/NetBeansProjects/JavaFXApplication7/dist/JavaFXApplication7.jar!/javafxapplication7/Sample.fxml:13
  at javafx.fxml.FXMLLoader$Element.processEventHandlerAttributes(Unknown Source)
  at javafx.fxml.FXMLLoader$ValueElement.processEndElement(Unknown Source)
  at javafx.fxml.FXMLLoader.processEndElement(Unknown Source)
  at javafx.fxml.FXMLLoader.load(Unknown Source)
  at javafx.fxml.FXMLLoader.load(Unknown Source)
  at javafx.fxml.FXMLLoader.load(Unknown Source)
  at javafx.fxml.FXMLLoader.load(Unknown Source)
  at javafx.fxml.FXMLLoader.load(Unknown Source)
  at javafx.fxml.FXMLLoader.load(Unknown Source)
  at javafx.fxml.FXMLLoader.load(Unknown Source)
  at javafxapplication7.JavaFXApplication7.start(JavaFXApplication7.java:21)
  at com.sun.javafx.application.LauncherImpl$5.run(Unknown Source)
  at com.sun.javafx.application.PlatformImpl$5.run(Unknown Source)
  at com.sun.javafx.application.PlatformImpl$4.run(Unknown Source)
  at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
  at com.sun.glass.ui.win.WinApplication.access$100(Unknown Source)
  at com.sun.glass.ui.win.WinApplication$2$1.run(Unknown Source)
  at java.lang.Thread.run(Thread.java:722)

Exception in Application start method
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at com.javafx.main.Main.launchApp(Main.java:642)
at com.javafx.main.Main.main(Main.java:805)
Caused by: java.lang.RuntimeException: Exception in Application start method
at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
at com.sun.javafx.application.LauncherImpl.access$000(Unknown Source)
at com.sun.javafx.application.LauncherImpl$1.run(Unknown Source)
at java.lang.Thread.run(Thread.java:722)
Caused by: javafx.fxml.LoadException: Page language not specified.
at javafx.fxml.FXMLLoader$Element.processEventHandlerAttributes(Unknown Source)
at javafx.fxml.FXMLLoader$ValueElement.processEndElement(Unknown Source)
at javafx.fxml.FXMLLoader.processEndElement(Unknown Source)
at javafx.fxml.FXMLLoader.load(Unknown Source)
at javafx.fxml.FXMLLoader.load(Unknown Source)
at javafx.fxml.FXMLLoader.load(Unknown Source)
at javafx.fxml.FXMLLoader.load(Unknown Source)
at javafx.fxml.FXMLLoader.load(Unknown Source)
at javafx.fxml.FXMLLoader.load(Unknown Source)
at javafx.fxml.FXMLLoader.load(Unknown Source)
at javafxapplication7.JavaFXApplication7.start(JavaFXApplication7.java:21)
at com.sun.javafx.application.LauncherImpl$5.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl$5.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl$4.run(Unknown Source)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(Unknown Source)
at com.sun.glass.ui.win.WinApplication$2$1.run(Unknown Source)
... 1 more
Java Result: 1