### 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)

has_filtered_items = true;
					data_type_has_filtered_items = true;
				}
				filtered_names.push_back(F);
			}
			if (filtered_names.size() == 0) {
				continue;
			}
			TreeItem *data_type_node = import_items_tree->create_item(type_node);
			data_type_node->set_meta("_can_be_imported", false);
			data_type_node->set_metadata(0, i);
			data_type_node->set_collapsed(!data_type_has_filtered_items);
			data_type_node->set_cell_mode(IMPORT_ITEM, TreeItem::CELL_MODE_CHECK);
			data_type_node->set_checked(IMPORT_ITEM, false);
			data_type_node->set_editable(IMPORT_ITEM, true);
			data_type_node->set_cell_mode(IMPORT_ITEM_DATA, TreeItem::CELL_MODE_CHECK);
			data_type_node->set_checked(IMPORT_ITEM_DATA, false);
			data_type_node->set_editable(IMPORT_ITEM_DATA, true);
			List<TreeItem *> *item_list = nullptr;
			switch (dt) {
				case Theme::DATA_TYPE_COLOR:
					data_type_node->set_icon(0, get_theme_icon(SNAME("Color"), SNAME("EditorIcons")));
					data_type_node->set_text(0, TTR("Colors"));
					item_list = &tree_color_items;
					color_amount += filtered_names.size();
					break;
				case Theme::DATA_TYPE_CONSTANT:
					data_type_node->set_icon(0, get_theme_icon(SNAME("MemberConstant"), SNAME("EditorIcons")));
					data_type_node->set_text(0, TTR("Constants"));
					item_list = &tree_constant_items;