Use-after-free when closing editor while using GDExtension that has registered classes

[mihe]

Tested versions

Reproducible in: 4.3.rc [739019e]

System information

Windows 11 (10.0.22631)

Issue description

As far as I can tell this looks to be a bug/regression introduced in #83747. I only noticed it when running the editor under Application Verifier on Windows, which similar to Valgrind lets you know if a use-after-free has occurred, among other things.

I haven't noticed any crash when running normally, but I suppose that could maybe happen, given the somewhat random nature of a use-after-free.

Basically, when closing the editor while a GDExtension (using godot-cpp) is loaded, that has itself registered a class with ClassDB, it will seemingly trigger a use-after-free here:

godot/editor/editor_help.cpp

Lines 2879 to 2881 in 739019e

	if (doc && doc->has_doc(p_class)) {
	doc->remove_doc(p_class);
	}

... as doc will already have been freed at that point, but not zeroed out.

The flow goes something like this:

1. Shutdown of the editor is initiated
2. SceneTree::finalize is called
3. EditorNode is deleted
4. EditorNode::~EditorNode calls EditorHelp::cleanup_doc
5. ⚠️ EditorHelp::cleanup_doc deletes EditorHelp::doc (but doesn't zero it out)
6. Main::cleanup is called (later down the line)
7. Main::cleanup calls GDExtensionManager::deinitialize_extensions (with INITIALIZATION_LEVEL_EDITOR in my case)
8. godot-cpp calls its ClassDB::deinitialize
9. ClassDB::deinitialize calls into Godot's GDExtension::_unregister_extension_class
10. GDExtension::_unregister_extension_class calls GDExtensionEditorHelp::remove_class
11. GDExtensionEditorHelp::remove_class calls EditorHelp::remove_class
12. Use-after-free occurs

Simply adding a doc = nullptr in EditorHelp::cleanup_doc seems to resolve the issue.

Steps to reproduce

N/A

Minimal reproduction project (MRP)

I'd be happy to provide an MRP, but I'm fairly sure this should be reproducible in pretty much any godot-cpp project.

[mihe]

I put up a tentative fix in #95307.

CC @dsnopek @Riteo