Fix unknown mod test failure

osu.Game/Screens/Play/SubmittingPlayer.cs

@@ -47,6 +47,12 @@ namespace osu.Game.Screens.Play
         [BackgroundDependencyLoader]
         private void load()
         {
+            if (DrawableRuleset == null)
+            {
+                // base load must have failed (e.g. due to an unknown mod); bail.
+                return;
+            }
+
             AddInternal(new PlayerTouchInputHandler());
         }