Fix editor crash when inspecting 2 objects handled by the same plugin

[Rindbee]

Already activated plugins do not need to be added again to editor_plugins_over.

fold_resource() changes active_plugins and is unsafe to call while iterating over active_plugins.

Fix #103909, fix #104250, fix #97577.
Close #103976.

[0xcafeb33f]

I tested this PR, and it fixes #103909, #104250, and #97577 on my setup (64-bit Windows).

[KoBeWi]

When you open a Shader from ShaderMaterial, the material gets folded and you can no longer unfold it.

godot.windows.editor.dev.x86_64_mcFfdq1NlV.mp4

In 4.5 dev1 trying to open it causes a crash 🙃

[Rindbee]

I couldn't reproduce it.

001.mp4

Tried from v4.5.dev1.official [97241ff], also only error message, no crash. Can you provide the MRP? That sounds like #103976.

[KoBeWi]

Try with built-in shader.
As for the crash, pretty sure it's random like #103909.

[Rindbee]

Try with built-in shader.

Still can't reproduce.

[0xcafeb33f]

Try with built-in shader.

I tried with a built-in shader on 4.5dev1 both with and without this PR, and I could not reproduce this issue. What type of node are you adding the material to? Is there anything else open in the editor, or present in the project, at the time of the bug occurring?

[KoBeWi]

I can't reproduce it anymore, idk what happened. Maybe I reloaded the scene and the shader wasn't closed properly for some reason (normally all built-in shaders are closed with the scene).

The error/crash is probably the same case as #104250, i.e. unfold a shader and restart editor. It's fixed by this PR.

[KoBeWi]

I tested various known errors when editing Resources in the inspector and they seem to be all gone. The ShaderMaterial problem I found might not have been caused by this PR, I didn't spot other problems.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.4.1.