Fix missing nullability consideraition

Author: peppy

osu.Game.Tests/Visual/UserInterface/TestSceneFirstRunSetupOverlay.cs

@@ -194,7 +194,7 @@ public void TestResumeViaNotification()
 
             AddStep("notification arrived", () => notificationOverlay.Verify(n => n.Post(It.IsAny<Notification>()), Times.Once));
 
-            AddStep("run notification action", () => lastNotification.Activated());
+            AddStep("run notification action", () => lastNotification.Activated?.Invoke());
 
             AddAssert("overlay shown", () => overlay.State.Value == Visibility.Visible);
             AddAssert("is resumed", () => overlay.CurrentScreen is ScreenUIScale);