[QoL] Split screen size change on tablet support

[TalalMash]

Crash log:

Android version: 10
Device: samsung SM-T800
Logs:
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setBackgroundColor(int)' on a null object reference
	at ru.ytkab0bp.slicebeam.navigation.Fragment.onApplyTheme(Fragment.java:37)
	at ru.ytkab0bp.slicebeam.navigation.NavigationDelegate.onApplyTheme(NavigationDelegate.java:57)
	at ru.ytkab0bp.slicebeam.navigation.MobileNavigationDelegate.onApplyTheme(MobileNavigationDelegate.java:40)
	at ru.ytkab0bp.slicebeam.MainActivity.onApplyTheme(MainActivity.java:820)
	at ru.ytkab0bp.slicebeam.theme.ThemesRepo.invalidate(ThemesRepo.java:38)
	at ru.ytkab0bp.slicebeam.MainActivity.onConfigurationChanged(MainActivity.java:815)
	at android.app.ActivityThread.performActivityConfigurationChanged(ActivityThread.java:5525)
	at android.app.ActivityThread.performConfigurationChangedForActivity(ActivityThread.java:5392)
	at android.app.ActivityThread.performConfigurationChangedForActivity(ActivityThread.java:5370)
	at android.app.ActivityThread.handleActivityConfigurationChanged(ActivityThread.java:5790)
	at android.app.ActivityThread$ActivityClientRecord.lambda$init$0$ActivityThread$ActivityClientRecord(ActivityThread.java:573)
	at android.app.-$$Lambda$ActivityThread$ActivityClientRecord$HOrG1qglSjSUHSjKBn2rXtX0gGg.onConfigurationChanged(Unknown Source:2)
	at android.view.ViewRootImpl.performConfigurationChange(ViewRootImpl.java:4396)
	at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2304)
	at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1742)
	at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7765)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:967)
	at android.view.Choreographer.doCallbacks(Choreographer.java:791)
	at android.view.Choreographer.doFrame(Choreographer.java:726)
	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:952)
	at android.os.Handler.handleCallback(Handler.java:883)
	at android.os.Handler.dispatchMessage(Handler.java:100)
	at android.os.Looper.loop(Looper.java:214)
	at android.app.ActivityThread.main(ActivityThread.java:7356)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:491)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:940)
    

Happens randomly when initiating split screen with the g-code viewer (after slice).

Great project, works fast and smooth on an 11 year old tablet.