### Godot version

v4.0.beta17.mono.official [c40020513]

### System information

Window 11, Forward+, GTX 1060

### Issue description

Adding two theme variations in a row result to a  crash.

https://user-images.githubusercontent.com/93253123/217037249-1ce1d56d-beb6-4435-83ad-9f8a07fe0e96.mp4



### Steps to reproduce

1. Create a project.
2. Create a theme.
3. Double click to go in the theme editor.
4. Click on manage items buttons.
5. In the add type field type a name to add a type.
6. type another name to add a second type.
7. Crash.

### Minimal reproduction project

[ThemeVariationCrash.zip](https://github.com/godotengine/godot/files/10665863/ThemeVariationCrash.zip)

EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
	ur->create_action(TTR("Unpin Stylebox"));
	ur->add_do_method(this, "_unpin_leading_stylebox");
	ur->add_undo_method(this, "_pin_leading_stylebox", leading_stylebox.item_name, leading_stylebox.stylebox);
	ur->commit_action();
}
void ThemeTypeEditor::_unpin_leading_stylebox() {
	if (leading_stylebox.stylebox.is_valid()) {
		leading_stylebox.stylebox->disconnect("changed", callable_mp(this, &ThemeTypeEditor::_update_stylebox_from_leading));
	}
	LeadingStylebox leader;
	leader.pinned = false;
	leading_stylebox = leader;
	_update_type_items();
}
void ThemeTypeEditor::_update_stylebox_from_leading() {