[Bug] NullReferenceException Closing Window During Animation

[spencerbaer]

Description

On WPF, if the window is closed while an animation is running, then a NullReferenceException will be thrown.

Steps to Reproduce

1. Start an animation using one of the ScaleTo, TranslateTo, etc. extensions.
2. Close the window while the animation is active

Expected Behavior

The application exits without issue.

Actual Behavior

A NullReferenceException is thrown:

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=Xamarin.Forms.Platform.WPF
StackTrace:
at Xamarin.Forms.Platform.WPF.WPFPlatformServices.BeginInvokeOnMainThread(Action action)
at Xamarin.Forms.Internals.Ticker.Remove(Int32 handle)
at Xamarin.Forms.Tweener.Finalize()

Basic Information

• Version with issue: v4.1.0

• IDE: VS 2019

• Platform Target Frameworks:

  • WPF: .NET Framework 4.7.2

Reproduction Link

AnimationOnExitDemo.zip

Application has a long-running animation that activates when the window appears, so just close the window after it opens to reproduce the issue. UWP project is also included for comparison.

[jfversluis]

Tried the repro project and confirmed it happens. There is a UWP project in the repro which doesn't show this behavior.