Add xmldoc and throw a local exception on null background

osu.Game/Screens/BackgroundScreen.cs

@@ -34,6 +34,10 @@ protected override bool OnKeyDown(KeyDownEvent e)
             return false;
         }
 
+        /// <summary>
+        /// Apply arbitrary changes to this background in a thread safe manner.
+        /// </summary>
+        /// <param name="action">The operation to perform.</param>
         public void ApplyToBackground(Action<BackgroundScreen> action) => Schedule(() => action.Invoke(this));
 
         protected override void Update()

osu.Game/Screens/OsuScreen.cs

@@ -115,8 +115,6 @@ private void createDependencies(IReadOnlyDependencyContainer dependencies)
             Mods = screenDependencies.Mods;
         }
 
-        public void ApplyToBackground(Action<BackgroundScreen> action) => background.ApplyToBackground(action);
-
         /// <summary>
         /// The background created and owned by this screen. May be null if the background didn't change.
         /// </summary>
@@ -148,6 +146,18 @@ private void load(OsuGame osu, AudioManager audio)
             Activity.Value ??= InitialActivity;
         }
 
+        /// <summary>
+        /// Apply arbitrary changes to the current background screen in a thread safe manner.
+        /// </summary>
+        /// <param name="action">The operation to perform.</param>
+        public void ApplyToBackground(Action<BackgroundScreen> action)
+        {
+            if (background == null)
+                throw new InvalidOperationException("Attempted to apply to background before screen is pushed");
+
+            background.ApplyToBackground(action);
+        }
+
         public override void OnResuming(IScreen last)
         {
             if (PlayResumeSound)