Fix null reference in PrefabCollectionList sync

Syncing the prefab list in OnCreated was throwing a null reference exception due to the Sync method relying on the AssetDatabase, which is not guaranteed to be in a stable state during overlay creation. Moved the sync call to OnDisplayChanged so it only runs once the editor UI is fully initialised.

Author: FrayedFunction

Editor/Scripts/Overlays/PaletteOverlay.cs

@@ -26,8 +26,6 @@ public override void OnCreated()
         {   
             collapsedIcon = Resources.Load<Texture2D>($"Imgs/PaletteIcon");
 
-            PrefabCollectionList.Instance.Sync();
-
             Instance = this;
 
             gui = new PaletteGUI();
@@ -74,6 +72,7 @@ private void OnDisplayChanged(bool isDisplayed)
         {
             if (isDisplayed)
             {
+                PrefabCollectionList.Instance.Sync();
                 ToolContext.Instance.OnEnable();
             }
             else