FocusManager NullPointerException

[fgnm]

I have a NullPointerException when a VisImageButton is clicked and Spinner is focused. This is the stack trace:

Exception in thread "main" java.lang.RuntimeException: Actor: VisImageButton
|  Image: tool-transform
	at com.badlogic.gdx.scenes.scene2d.Actor.notify(Actor.java:203)
	at com.badlogic.gdx.scenes.scene2d.Actor.fire(Actor.java:159)
	at com.badlogic.gdx.scenes.scene2d.Stage.touchDown(Stage.java:283)
	at com.badlogic.gdx.InputMultiplexer.touchDown(InputMultiplexer.java:113)
	at com.badlogic.gdx.InputEventQueue.drain(InputEventQueue.java:83)
	at com.badlogic.gdx.backends.lwjgl3.DefaultLwjgl3Input.update(DefaultLwjgl3Input.java:208)
	at com.badlogic.gdx.backends.lwjgl3.Lwjgl3Window.update(Lwjgl3Window.java:394)
	at com.badlogic.gdx.backends.lwjgl3.Lwjgl3Application.loop(Lwjgl3Application.java:138)
	at com.badlogic.gdx.backends.lwjgl3.Lwjgl3Application.<init>(Lwjgl3Application.java:114)
	at games.rednblack.editor.Main.main(Main.java:40)
Caused by: java.lang.NullPointerException
	at com.kotcrab.vis.ui.FocusManager.switchFocus(FocusManager.java:42)
	at com.kotcrab.vis.ui.widget.VisImageButton$1.touchDown(VisImageButton.java:95)
	at com.badlogic.gdx.scenes.scene2d.InputListener.handle(InputListener.java:58)
	at com.badlogic.gdx.scenes.scene2d.Actor.notify(Actor.java:189)
	... 9 more

I don't know how hard is to reproduce, because in my project the crash happens only on some Spinner, expecially when they're placed in multiple nested VisTable wrapped from a ScrollPane. Reading source code it seams that focusedWidget is null for some reason L42.
Maybe a null check could fix the problem, please let me know if I can debug better the situation.
Thanks!

[kotcrab]

switchFocus (Stage stage, Focusable widget) is always called with this as the widget- it can't be null at that point. L95

resetFocus can set it to null but to cause this it would have to be done simultaneously by some other thread. Are you using multiple threads? FocusManager is not thread safe.

[kotcrab]

There is also a chance that stage.setKeyboardFocus somehow ended up calling resetFocus. I would need a test case to debug this.

[fgnm]

I've noticed that it happens only when the Spinner is placed in that particular nested position. I'll try to create a test case in order to reproduce.
Anyway, I'm not using multi thread. And the crash happens always to that Spinners

[fgnm]

With break points I've found the problem! And maybe nested stuff doesn't really matters. The problem is in stage.setKeyboardFocus.

1. I have a ChangeListener attached to the Spinner and during the stage.setKeyboardFocus(null) this listener is triggered
2. In changed method one of my VisTextField is set to disabled state, so this trigger resetFocus (Stage stage, Actor caller) and focusedWidget becomes null.
3. We have the crash.

Now,I can try to avoid that the ChangeListener triggers setDisabled on the other widgets and fix on my code, but even for further problems would be better to change FocusManager in order to prevent this kind of conditions. Please let me know

[kotcrab]

To add to this, the issue is triggered by the widget the focus is being switched to. It will call switchFocus with self, set the focusWidget to self and do stage.setKeyboardFocus(null) which will then trigger the change listeners on spinner (only if TextFieldEventPolicy.ON_FOCUS_LOST is used) and then in that change listener you must call some method which calls resetFocus.