Window-derived root causes scene thumbnail to get generated every time

[KoBeWi]

Tested versions

4.5 beta1

System information

W10

Issue description

When you have a scene with Window (or any dialog) as root, the editor will try to generate its preview all the time. The preview is attempted when opening the project and when saving the scene. Looks like the method skips generating the thumbnail and never writes to cache file, so the editor interprets it as if the thumbnail did not exist or was outdated.

Steps to reproduce

1. Create a scene with Window root
2. Save it
3. While there is no visible effect, you can put a print here
   

   godot/editor/plugins/editor_preview_plugins.cpp

   Line 319 in 8de08c7

   Ref<Texture2D> EditorPackedScenePreviewPlugin::generate_from_path(const String &p_path, const Size2 &p_size, Dictionary &p_metadata) const {

   
   to see the preview is re-generated every time you save

Minimal reproduction project (MRP)

N/A