Simulate a mouse click during test. JUnit, JavaFX

In my test method, I'm trying to simulate a mouse click using Robot class:

@Test
public void testMouseEvents(){
    clickMouse();
}

private void clickMouse() {
    final boolean[] flag = {true};
    Platform.runLater(() -> {
        try {
            Robot robot = new Robot();
            robot.mouseMove(900, 500);
            robot.delay(2000);
            robot.mousePress(InputEvent.BUTTON1_DOWN_MASK);
            robot.delay(2000);
            robot.mouseRelease(InputEvent.BUTTON1_DOWN_MASK);
            robot.delay(2000);
        } catch (Exception ignored) {

        }
        flag[0] = false;
    });

    try {
        while (flag[0]) {
            Thread.sleep(3);
        }
        Thread.sleep(20);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

But the last method throws different exeptions: sometimes it's NullPointerm but more often it's:

java.util.concurrent.RejectedExecutionException: Task     com.sun.javafx.tk.quantum.PaintRenderJob@24f7eafc rejected from  com.sun.javafx.tk.quantum.QuantumRenderer@14a5ef25[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 4]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2047)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:823)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1369)
at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:112)

Curious, but when I simulate KeyEvent, it works perfectly. Moreover, if I delete the line where the mouse key is supposed to be pressed, the robot moves the cursor and no exception is thrown. I suspect that the issue might concern the thread in which JavaFX application works. But still I can't figure out why it doesn't work.