### 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)

import_buttons->add_spacer();
	Button *import_add_selected_button = memnew(Button);
	import_add_selected_button->set_text(TTR("Import Selected"));
	import_buttons->add_child(import_add_selected_button);
	import_add_selected_button->connect("pressed", callable_mp(this, &ThemeItemImportTree::_import_selected));
}
void ThemeItemEditorDialog::ok_pressed() {
	if (import_default_theme_items->has_selected_items() || import_editor_theme_items->has_selected_items() || import_other_theme_items->has_selected_items()) {
		confirm_closing_dialog->set_text(TTR("Import Items tab has some items selected. Selection will be lost upon closing this window.\nClose anyway?"));
		confirm_closing_dialog->popup_centered(Size2(380, 120) * EDSCALE);
		return;
	}
	hide();
}
void ThemeItemEditorDialog::_close_dialog() {
	hide();
}
void ThemeItemEditorDialog::_dialog_about_to_show() {
	ERR_FAIL_COND_MSG(edited_theme.is_null(), "Invalid state of the Theme Editor; the Theme resource is missing.");
	_update_edit_types();
	import_default_theme_items->set_edited_theme(edited_theme);
	import_default_theme_items->set_base_theme(ThemeDB::get_singleton()->get_default_theme());
	import_default_theme_items->reset_item_tree();
	import_editor_theme_items->set_edited_theme(edited_theme);
	import_editor_theme_items->set_base_theme(EditorNode::get_singleton()->get_theme_base()->get_theme());
	import_editor_theme_items->reset_item_tree();
	import_other_theme_items->set_edited_theme(edited_theme);
	import_other_theme_items->reset_item_tree();
}
void ThemeItemEditorDialog::_update_edit_types() {