### 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("Set Color Item in Theme"), UndoRedo::MERGE_ENDS);
	ur->add_do_method(*edited_theme, "set_color", p_item_name, edited_type, p_value);
	ur->add_undo_method(*edited_theme, "set_color", p_item_name, edited_type, edited_theme->get_color(p_item_name, edited_type));
	ur->commit_action();
}
void ThemeTypeEditor::_constant_item_changed(float p_value, String p_item_name) {
	EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
	ur->create_action(TTR("Set Constant Item in Theme"));
	ur->add_do_method(*edited_theme, "set_constant", p_item_name, edited_type, p_value);
	ur->add_undo_method(*edited_theme, "set_constant", p_item_name, edited_type, edited_theme->get_constant(p_item_name, edited_type));
	ur->commit_action();
}
void ThemeTypeEditor::_font_size_item_changed(float p_value, String p_item_name) {
	EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
	ur->create_action(TTR("Set Font Size Item in Theme"));
	ur->add_do_method(*edited_theme, "set_font_size", p_item_name, edited_type, p_value);
	ur->add_undo_method(*edited_theme, "set_font_size", p_item_name, edited_type, edited_theme->get_font_size(p_item_name, edited_type));
	ur->commit_action();
}
void ThemeTypeEditor::_edit_resource_item(Ref<Resource> p_resource, bool p_edit) {