Platform.runLater(new Runnable() {
                    @Override public void run() {
                        Controller instance2 = new Controller();
                        instance2.startonpause();
                    }
                });

public void startonpause() {
            startonpause.setText("123");
}

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
    at sample.Controller$4.run(Controller.java:302)
    at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
    at java.lang.Thread.run(Thread.java:748)