| existing | An existing object to be persisted, or null to create a new object. If no persisted state is found, a non-null object will be returned as-is. |
| key | The key for the current VisualElement to be used with the persistence store on the EditorWindow. |
T The same object being passed in (or a new one if null was passed in), but possibly with its persistent state restored.
Takes a reference to an existing persisted object and a key and returns the object either filled with the persisted state or as-is.
Takes a reference to an existing (or null) persisted object and a key, ideally from VisualElement.GetFullHierarchicalPersistenceKey, and returns the object either filled with the persisted state or as-is. It will also create a new persisted object if its passed in null and no persisted state is found. This function will not save anything into the persistence store on the EditorWindow.