NRE in Animation API

[michael-hawker]

Describe the bug

Getting a Null Reference exception when navigating away from a sample page that is animating.

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

• Can this be reproduced in the Sample App? (Either in a sample as-is or with new XAML pasted in the editor.) If so, please provide custom XAML or steps to reproduce. If not, let us know why it can't be reproduced (e.g. more complex setup, environment, dependencies, etc...)

Steps to reproduce the behavior:

1. In Sample App
2. Go to Shadow Animations example
3. Click on the button to start the animation
4. While it is playing, click on home
5. Crash...

Looks like target is null here:

WindowsCommunityToolkit/Microsoft.Toolkit.Uwp.UI.Animations/Builders/NormalizedKeyFrameAnimationBuilder{T}.Composition.cs

Line 135 in fa96904

Vector3KeyFrameAnimation vector3Animation = target.Compositor.CreateVector3KeyFrameAnimation();

Expected behavior

Not crash

Screenshots

Environment

NuGet Package(s): Animation

Package Version(s): latest main

Windows 10 Build Number:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• May 2020 Update (19041)
• Insider Build ({build_number})

App min and target version:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• May 2020 Update (19041)
• Insider Build ({build_number})

Device form factor:

• Desktop
• Xbox
• Surface Hub
• IoT

Visual Studio version:

• 2017 (15.{minor_version})
• 2019 (16.{minor_version})
• 2022 (17.{minor_version})

Additional context

FYI @Sergio0694