### 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)

add_child(import_buttons);
	import_collapse_types_button = memnew(Button);
	import_collapse_types_button->set_flat(true);
	import_collapse_types_button->set_tooltip_text(TTR("Collapse types."));
	import_buttons->add_child(import_collapse_types_button);
	import_collapse_types_button->connect("pressed", callable_mp(this, &ThemeItemImportTree::_toggle_type_items).bind(true));
	import_expand_types_button = memnew(Button);
	import_expand_types_button->set_flat(true);
	import_expand_types_button->set_tooltip_text(TTR("Expand types."));
	import_buttons->add_child(import_expand_types_button);
	import_expand_types_button->connect("pressed", callable_mp(this, &ThemeItemImportTree::_toggle_type_items).bind(false));
	import_buttons->add_child(memnew(VSeparator));
	import_select_all_button = memnew(Button);
	import_select_all_button->set_flat(true);
	import_select_all_button->set_text(TTR("Select All"));
	import_select_all_button->set_tooltip_text(TTR("Select all Theme items."));
	import_buttons->add_child(import_select_all_button);
	import_select_all_button->connect("pressed", callable_mp(this, &ThemeItemImportTree::_select_all_items_pressed));
	import_select_full_button = memnew(Button);
	import_select_full_button->set_flat(true);
	import_select_full_button->set_text(TTR("Select With Data"));
	import_select_full_button->set_tooltip_text(TTR("Select all Theme items with item data."));
	import_buttons->add_child(import_select_full_button);
	import_select_full_button->connect("pressed", callable_mp(this, &ThemeItemImportTree::_select_full_items_pressed));
	import_deselect_all_button = memnew(Button);
	import_deselect_all_button->set_flat(true);
	import_deselect_all_button->set_text(TTR("Deselect All"));
	import_deselect_all_button->set_tooltip_text(TTR("Deselect all Theme items."));
	import_buttons->add_child(import_deselect_all_button);
	import_deselect_all_button->connect("pressed", callable_mp(this, &ThemeItemImportTree::_deselect_all_items_pressed));