Fixed a crash with the items list animations in Sample app (#3801)

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## Fixes #3701
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## PR Type
What kind of change does this PR introduce?
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- Bugfix (sample app)
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
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<!-- - Sample app changes -->
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## What is the current behavior?
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There is a crash in the items animations when looking for samples. This was unrelated from the actual animation APIs, and the crash was happening because the parent of the `DropShadowPanel` for each item was actually `null` if the items were being swapped by a text change while one was animated, so the animation was being invoked on `null`, resulting in a crash. I've just added two checks to skip the animation out in case the whole list is being refreshed, as the item containers would all be discarded anyway.

## What is the new behavior?
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The crash is fixed.

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [X] New major technical changes in the toolkit have or will be added to the [Wiki](https://github.com/windows-toolkit/WindowsCommunityToolkit/wiki) e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

Microsoft.Toolkit.Uwp.SampleApp/Shell.SamplePicker.cs

@@ -230,25 +230,29 @@ private void ContainerItem_Loaded(object sender, RoutedEventArgs e)
 
         private void ItemContainer_PointerExited(object sender, Windows.UI.Xaml.Input.PointerRoutedEventArgs e)
         {
-            var panel = (sender as FrameworkElement).FindDescendant<DropShadowPanel>();
-            if (panel != null)
+            if ((sender as FrameworkElement)?.FindDescendant<DropShadowPanel>() is FrameworkElement panel)
             {
                 AnimationBuilder.Create().Opacity(0, duration: TimeSpan.FromMilliseconds(1200)).Start(panel);
-                AnimationBuilder.Create().Scale(1, duration: TimeSpan.FromMilliseconds(1200)).Start((UIElement)panel.Parent);
+
+                if (panel.Parent is UIElement parent)
+                {
+                    AnimationBuilder.Create().Scale(1, duration: TimeSpan.FromMilliseconds(1200)).Start(parent);
+                }
             }
         }
 
         private void ItemContainer_PointerEntered(object sender, Windows.UI.Xaml.Input.PointerRoutedEventArgs e)
         {
-            if (e.Pointer.PointerDeviceType == Windows.Devices.Input.PointerDeviceType.Mouse)
+            if (e.Pointer.PointerDeviceType == Windows.Devices.Input.PointerDeviceType.Mouse &&
+                (sender as FrameworkElement)?.FindDescendant<DropShadowPanel>() is FrameworkElement panel)
             {
-                var panel = (sender as FrameworkElement).FindDescendant<DropShadowPanel>();
-                if (panel != null)
-                {
-                    panel.Visibility = Visibility.Visible;
+                panel.Visibility = Visibility.Visible;
+
+                AnimationBuilder.Create().Opacity(1, duration: TimeSpan.FromMilliseconds(600)).Start(panel);
 
-                    AnimationBuilder.Create().Opacity(1, duration: TimeSpan.FromMilliseconds(600)).Start(panel);
-                    AnimationBuilder.Create().Scale(1.1, duration: TimeSpan.FromMilliseconds(600)).Start((UIElement)panel.Parent);
+                if (panel.Parent is UIElement parent)
+                {
+                    AnimationBuilder.Create().Scale(1.1, duration: TimeSpan.FromMilliseconds(600)).Start(parent);
                 }
             }
         }