[3.4] Theme Editor crash when clicking the element picker

[Yagich]

Godot version

3.4

System information

Ubuntu 21.10

Issue description

The new theme editor in 3.4 crashes the editor when trying to use the element picker in a custom scene selected as preview after refreshing.
cc @pycbouh

Steps to reproduce

1. Create or edit a Theme resource
2. In the Theme Editor dock, add a preview scene with some Control nodes
3. Click the picker button and select an element
4. Click the "Reload the scene to reflect its most actual state" button
5. Click the picker again

Minimal reproduction project

No response