Fix potential crash when losing network connectivity at online play screen

Addresses #20448.

osu.Game/Screens/OnlinePlay/OnlinePlayScreen.cs

@@ -105,7 +105,8 @@ private void forcefullyExit()
                 while (this.IsCurrentScreen())
                     this.Exit();
             }
-            else
+            // Also handle the case where a child screen is current (ie. gameplay).
+            else if (this.GetChildScreen() != null)
             {
                 this.MakeCurrent();
                 Schedule(forcefullyExit);