Fix default beatmap not being correctly set after aborting new beatmap creation

[peppy]

---

Closes #32337.

Also went down the path of ensuring we never have a placeholder track selected (https://github.com/ppy/osu/compare/master...peppy:osu:never-placeholder-beatmap?expand=1) but then saw that this case was already handled in the fail scenarios, and we were just incorrectly dereferencing the default beatmap instance via cache weirdness.

[bdach]

I'm not sure what it is precisely but I'm deeply uncomfortable with this id check. DefaultBeatmap's ID isn't even fixed to some constant value, it just looks to be getting a random Guid.NewGuid() via the default BeatmapInfo ctor.

Is there any reason we can't go with something like the following instead?

diff --git a/osu.Game/Screens/Edit/Editor.cs b/osu.Game/Screens/Edit/Editor.cs
index f56380a34d..925dd169e9 100644
--- a/osu.Game/Screens/Edit/Editor.cs
+++ b/osu.Game/Screens/Edit/Editor.cs
@@ -915,6 +915,9 @@ public override void OnSuspending(ScreenTransitionEvent e)
 
         private void refetchBeatmap()
         {
+            if (Beatmap.IsDefault)
+                return;
+
             // To update the game-wide beatmap with any changes, perform a re-fetch on exit/suspend.
             // This is required as the editor makes its local changes via EditorBeatmap
             // (which are not propagated outwards to a potentially cached WorkingBeatmap).

[peppy]

We could do that as well, but I'd want to guard in WorkingBeatmapCache as well, since this is bound to occur via some other means. Right now, doing a GetWorkingBeatmap call will have a chance of converting a default/dummy beatmap into something that resembles it, but isn't considered default.

diff --git a/osu.Game/Beatmaps/WorkingBeatmapCache.cs b/osu.Game/Beatmaps/WorkingBeatmapCache.cs
index bd125deddf..30bbbbc1fe 100644
--- a/osu.Game/Beatmaps/WorkingBeatmapCache.cs
+++ b/osu.Game/Beatmaps/WorkingBeatmapCache.cs
@@ -89,7 +89,7 @@ public void Invalidate(BeatmapInfo info)
 
         public virtual WorkingBeatmap GetWorkingBeatmap([CanBeNull] BeatmapInfo beatmapInfo)
         {
-            if (beatmapInfo?.ID == DefaultBeatmap.BeatmapInfo.ID || beatmapInfo?.BeatmapSet == null)
+            if (beatmapInfo == null || ReferenceEquals(beatmapInfo, DefaultBeatmap.BeatmapInfo))
                 return DefaultBeatmap;
 
             lock (workingCache)

I'm guessing this will sit better with you? It seems to work just as well (waiting on tests to confirm that we don't need to check BeatmapSet for null, but it seems really weird that we would need to be).

[bdach]

If it works then I suppose I could live with a reference equality check sure. Seems less prone to accidental breakage.