Fix rare and strange edge case that can cause GUIDs to fail to save

I detect when a guid is getting created for a Prefab Instance and guarantee that we call RecordPrefabInstanceModifications even if the prefab parent is invalid.

Author: william-w-armstrong

Assets/CrossSceneReference/GuidComponent.cs

@@ -29,6 +29,16 @@ void CreateGuid()
         {
             guid = System.Guid.NewGuid();
             serializedGuid = guid.ToByteArray();
+
+if UNITY_EDITOR
+            // If we are creating a new GUID for a prefab instance of a prefab, but we have somehow lost our prefab connection
+            // force a save of the modified prefab instance properties
+            PrefabType prefabType = PrefabUtility.GetPrefabType(this);
+            if (prefabType == PrefabType.PrefabInstance)
+            {
+                PrefabUtility.RecordPrefabInstancePropertyModifications(this);
+            }
+#endif
         }
         else if (guid == System.Guid.Empty)
         {