### 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)

if (keying == p_active) {
		return;
	}
	keying = p_active;
	update_tree();
}
void EditorInspector::set_read_only(bool p_read_only) {
	read_only = p_read_only;
	update_tree();
}
EditorPropertyNameProcessor::Style EditorInspector::get_property_name_style() const {
	return property_name_style;
}
void EditorInspector::set_property_name_style(EditorPropertyNameProcessor::Style p_style) {
	if (property_name_style == p_style) {
		return;
	}
	property_name_style = p_style;
	update_tree();
}
void EditorInspector::set_autoclear(bool p_enable) {
	autoclear = p_enable;
}
void EditorInspector::set_show_categories(bool p_show) {
	show_categories = p_show;
	update_tree();
}
void EditorInspector::set_use_doc_hints(bool p_enable) {