Fix Peek video player showing black screen after viewing multiple videos

[Copilot]

Issue

When using Peek to preview video files, after viewing approximately three videos, subsequent videos would show only a black screen instead of the expected video content. The video controls would appear but the content area would remain black.

Root Cause

The MediaPlayer object in the VideoPreview control wasn't being properly reset between video previews. After viewing multiple videos, the player would get into an inconsistent state, resulting in a black screen for subsequent videos.

Solution

The fix properly disposes of and recreates the MediaPlayer object between video previews:

1. Added a null check for VideoPreview.MediaPlayer
2. Updated the resource management in OnPreviewerChanging method to:
   • Pause the current player
   • Clear the current source
   • Store a reference to the old player
   • Create a new MediaPlayer instance with SetMediaPlayer
   • Dispose of the old player to release resources

This ensures each video preview starts with a fresh MediaPlayer instance, preventing the black screen issue after viewing multiple videos.

Fixes #9.

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