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)