chore: fix stop multiple times error

src/AddIns/Uno.UI.MediaPlayer.Skia.Gtk/MediaPlayerExtension.cs

@@ -143,7 +143,16 @@ public partial class MediaPlayerExtension : IMediaPlayerExtension
 
 	public void Pause()
 	{
-		if (_owner.PlaybackSession.PlaybackState == MediaPlaybackState.Playing)
+		if (_owner.PlaybackSession.PlaybackState == MediaPlaybackState.Playing
+				&& _player != null
+				&& _player.CurrentState == MediaPlayerState.Playing)
+		{
+			_player?.Pause();
+			_owner.PlaybackSession.PlaybackState = MediaPlaybackState.Paused;
+		}
+		if (_owner.PlaybackSession.PlaybackState == MediaPlaybackState.Paused
+				&& _player != null
+				&& _player.CurrentState != MediaPlayerState.Paused)
 		{
 			_player?.Pause();
 			_owner.PlaybackSession.PlaybackState = MediaPlaybackState.Paused;

src/Uno.UI/UI/Xaml/Controls/MediaPlayerElement/MediaTransportControls.MediaPlayer.cs

@@ -231,8 +231,8 @@ namespace Windows.UI.Xaml.Controls
 		{
 			_skipPlayPauseStateUpdate = false;
 
-			_mediaPlayer?.Pause();
 			ResetProgressSlider();
+			_mediaPlayer?.Pause();
 			_mediaPlayer?.Stop();
 		}