Fix editor potentially using a track post-disposal

This changes the editor to track the current track as it is *loaded* by
`MusicController`, rather than haphazardly following the current global
`WorkingBeatmap` (with a potentially unloaded track) or relying on local
immediate-load behaviour (as implemented in `ResourcesSection`).

osu.Game/Overlays/MusicController.cs

@@ -70,7 +70,11 @@ private void load()
         /// <summary>
         /// Forcefully reload the current <see cref="WorkingBeatmap"/>'s track from disk.
         /// </summary>
-        public void ReloadCurrentTrack() => changeTrack();
+        public void ReloadCurrentTrack()
+        {
+            changeTrack();
+            TrackChanged?.Invoke(current, TrackChangeDirection.None);
+        }
 
         /// <summary>
         /// Returns whether the beatmap track is playing.

osu.Game/Screens/Edit/Editor.cs

@@ -329,6 +329,9 @@ private void load(OsuConfigManager config)
             changeHandler?.CanRedo.BindValueChanged(v => redoMenuItem.Action.Disabled = !v.NewValue, true);
         }
 
+        [Resolved]
+        private MusicController musicController { get; set; }
+
         protected override void LoadComplete()
         {
             base.LoadComplete();
@@ -336,12 +339,18 @@ protected override void LoadComplete()
 
             Mode.Value = isNewBeatmap ? EditorScreenMode.SongSetup : EditorScreenMode.Compose;
             Mode.BindValueChanged(onModeChanged, true);
+
+            musicController.TrackChanged += onTrackChanged;
         }
 
-        /// <summary>
-        /// If the beatmap's track has changed, this method must be called to keep the editor in a valid state.
-        /// </summary>
-        public void UpdateClockSource() => clock.ChangeSource(Beatmap.Value.Track);
+        protected override void Dispose(bool isDisposing)
+        {
+            base.Dispose(isDisposing);
+
+            musicController.TrackChanged -= onTrackChanged;
+        }
+
+        private void onTrackChanged(WorkingBeatmap working, TrackChangeDirection direction) => clock.ChangeSource(working.Track);
 
         /// <summary>
         /// Creates an <see cref="EditorState"/> instance representing the current state of the editor.

osu.Game/Screens/Edit/Setup/ResourcesSection.cs

@@ -118,8 +118,6 @@ public bool ChangeAudioTrack(FileInfo source)
             working.Value.Metadata.AudioFile = destination.Name;
 
             music.ReloadCurrentTrack();
-
-            editor?.UpdateClockSource();
             return true;
         }