Fix being able to trigger player before carousel is ready

Causes an eventual crash.

osu.Game/Screens/Select/SongSelect.cs

@@ -231,6 +231,10 @@ public void Edit(BeatmapInfo beatmap)
         /// <param name="performStartAction">Whether to trigger <see cref="OnStart"/>.</param>
         public void FinaliseSelection(BeatmapInfo beatmap = null, bool performStartAction = true)
         {
+            // avoid attempting to continue before a selection has been obtained.
+            // this could happen via a user interaction while the carousel is still in a loading state.
+            if (Carousel.SelectedBeatmap == null) return;
+
             // if we have a pending filter operation, we want to run it now.
             // it could change selection (ie. if the ruleset has been changed).
             Carousel.FlushPendingFilterOperations();