*+Description+*
The VisualTreeAsset of a new UIBuilder is destroyed after entering playmode and never restored.

*+Initial setup+*
None

*+Reproduction steps+*
1. Open the UIBuilder

2. Create a new document

3. Add a VisualElement

4. Enter play mode

5. Stop play mode

6. Try to add a new VisualElement

*+Expected behaviour+*
Everything works are normally. The VisualElement is not greyed out in the Hierarchy view and the inspector display all the attributes of the selected VisualElement.

*+Actual behavior+*
4. The VisualElement is greyed out in the Hierarchy view. In the inspector, the attributes are lost.

6. An exception occurs when trying to add a new VisualElement because there is no VisualTreeAsset anymore. 

*+Logs and Links+*
(Add links to Yamato jobs, links to logs, and add a stack trace in this section)