XF.UWP crash when navigating back

## 🐛 Bug Report

I have a XF.UWP application.

I am currently using v4.0.11, because with v4.1.0 it does not render any animation on UWP (I will file a separate bug report for that)

v4.0.11 can trigger a crash (unhandled nullref exception)
This happens when I quickly navigate away from a page with an Lottie animation.
On UWP it seems to need approx 200ms to start animating the visual after displaying the 'begin state'.
If I quickly navigate back (for instance with the back button on my mouse), and I am within this 200ms (the animation has not started yet), the app crashes with a nullref exception.
If I wait for the animation to start moving, and move back then, all is fine.

### Expected behavior
no crash.

### Reproduction steps

### Configuration

**Version:** 1.x

**Platform:**
- [ ] :iphone: iOS
- [ ] :robot: Android
- [x ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms

No source files, tests, or reproduction project are named. Start by reproducing the crash on UWP with LottieXamarin v4.0.11, navigating back within about 200 ms of displaying an animation's begin state. Trace the animation teardown and navigation path; done means rapid back navigation no longer causes an unhandled null reference.

Written by the indexing model from the issue text.