Fix regression in screen change allowance logic

osu.Game/OsuGame.cs

@@ -292,7 +292,7 @@ namespace osu.Game
                 return;
             }
 
-            if ((screenStack.CurrentScreen as IOsuScreen)?.DisallowExternalBeatmapRulesetChanges != false)
+            if ((screenStack.CurrentScreen as IOsuScreen)?.AllowExternalScreenChange == false)
             {
                 notifications.Post(new SimpleNotification
                 {

osu.Game/Screens/IOsuScreen.cs

@@ -17,6 +17,12 @@ namespace osu.Game.Screens
         /// </summary>
         bool DisallowExternalBeatmapRulesetChanges { get; }
 
+        /// <summary>
+        /// Whether a top-level component should be allowed to exit the current screen to, for example,
+        /// complete an import.
+        /// </summary>
+        bool AllowExternalScreenChange { get; }
+
         /// <summary>
         /// Whether this <see cref="OsuScreen"/> allows the cursor to be displayed.
         /// </summary>

osu.Game/Screens/Select/MatchSongSelect.cs

@@ -17,6 +17,8 @@ namespace osu.Game.Screens.Select
         public string ShortTitle => "song selection";
         public override string Title => ShortTitle.Humanize();
 
+        public override bool AllowExternalScreenChange => false;
+
         public MatchSongSelect()
         {
             Padding = new MarginPadding { Horizontal = HORIZONTAL_OVERFLOW_PADDING };