Fix crash from being able to perform selection after entering play mode

Closes #1757

osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmap.cs

@@ -46,7 +46,7 @@ namespace osu.Game.Screens.Select.Carousel
         {
             if (songSelect != null)
             {
-                startRequested = songSelect.Start;
+                startRequested = songSelect.FinaliseSelection;
                 editRequested = songSelect.Edit;
             }
 

osu.Game/Screens/Select/EditSongSelect.cs

@@ -7,6 +7,10 @@ namespace osu.Game.Screens.Select
     {
         protected override bool ShowFooter => false;
 
-        protected override void Start() => Exit();
+        protected override bool OnSelectionFinalised()
+        {
+            Exit();
+            return true;
+        }
     }
 }

osu.Game/Screens/Select/MatchSongSelect.cs

@@ -5,6 +5,10 @@ namespace osu.Game.Screens.Select
 {
     public class MatchSongSelect : SongSelect
     {
-        protected override void Start() => Exit();
+        protected override bool OnSelectionFinalised()
+        {
+            Exit();
+            return true;
+        }
     }
 }

osu.Game/Screens/Select/PlaySongSelect.cs

@@ -124,9 +124,9 @@ namespace osu.Game.Screens.Select
             return false;
         }
 
-        protected override void Start()
+        protected override bool OnSelectionFinalised()
         {
-            if (player != null) return;
+            if (player != null) return false;
 
             // Ctrl+Enter should start map with autoplay enabled.
             if (GetContainingInputManager().CurrentState?.Keyboard.ControlPressed == true)
@@ -147,7 +147,12 @@ namespace osu.Game.Screens.Select
 
             sampleConfirm?.Play();
 
-            LoadComponentAsync(player = new PlayerLoader(new Player()), l => Push(player));
+            LoadComponentAsync(player = new PlayerLoader(new Player()), l =>
+            {
+                if (IsCurrentScreen) Push(player);
+            });
+
+            return true;
         }
     }
 }

osu.Game/Screens/Select/SongSelect.cs

@@ -227,12 +227,17 @@ namespace osu.Game.Screens.Select
             Push(new Editor());
         }
 
-        public void Start(BeatmapInfo beatmap)
+        /// <summary>
+        /// Call to make a selection and perform the default action for this SongSelect.
+        /// </summary>
+        /// <param name="beatmap">An optional beatmap to override the current carousel selection.</param>
+        public void FinaliseSelection(BeatmapInfo beatmap = null)
         {
             // 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();
 
+            if (beatmap != null)
                 Carousel.SelectBeatmap(beatmap);
 
             if (selectionChangedDebounce?.Completed == false)
@@ -242,13 +247,14 @@ namespace osu.Game.Screens.Select
                 selectionChangedDebounce = null;
             }
 
-            Start();
+            OnSelectionFinalised();
         }
 
         /// <summary>
         /// Called when a selection is made.
         /// </summary>
-        protected abstract void Start();
+        /// <returns>If a resultant action occurred that takes the user away from SongSelect.</returns>
+        protected abstract bool OnSelectionFinalised();
 
         private ScheduledDelegate selectionChangedDebounce;
 
@@ -339,7 +345,7 @@ namespace osu.Game.Screens.Select
 
             logo.Action = () =>
             {
-                Start();
+                FinaliseSelection();
                 return false;
             };
         }
@@ -462,7 +468,7 @@ namespace osu.Game.Screens.Select
             {
                 case Key.KeypadEnter:
                 case Key.Enter:
-                    Start();
+                    FinaliseSelection();
                     return true;
                 case Key.Delete:
                     if (state.Keyboard.ShiftPressed)