Fix `MultiplayerMatchSongSelect` potentially attempting to exit when not the current screen

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerMatchSongSelect.cs

@@ -80,6 +80,10 @@ protected override void SelectItem(PlaylistItem item)
                 {
                     Schedule(() =>
                     {
+                        // If an error or server side trigger occurred this screen may have already exited by external means.
+                        if (!this.IsCurrentScreen())
+                            return;
+
                         loadingLayer.Hide();
 
                         if (t.IsFaulted)