Fix MediaPlayer property null for #106 and #68

Expose internal property which has correct reference, as was done for WinForms.

Microsoft.Toolkit.Forms.UI.Controls/MediaPlayerElement/MediaPlayerElement.cs

@@ -153,7 +153,7 @@ namespace Microsoft.Toolkit.Forms.UI.Controls
         [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
         public MediaPlayer MediaPlayer
         {
-            get => UwpControl.MediaPlayer;
+            get => UwpControl?.MediaPlayer;
         }
     }
 }

Microsoft.Toolkit.Wpf.UI.Controls/MediaPlayerElement/MediaPlayerElement.cs

@@ -179,7 +179,7 @@ namespace Microsoft.Toolkit.Wpf.UI.Controls
         /// </summary>
         public MediaPlayer MediaPlayer
         {
-            get => (MediaPlayer)GetValue(MediaPlayerProperty);
+            get => UwpControl?.MediaPlayer;
         }
     }
 }