### 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)

bool keying = false;
	bool deletable = false;
	Rect2 right_child_rect;
	Rect2 bottom_child_rect;
	Rect2 keying_rect;
	bool keying_hover = false;
	Rect2 revert_rect;
	bool revert_hover = false;
	Rect2 check_rect;
	bool check_hover = false;
	Rect2 delete_rect;
	bool delete_hover = false;
	bool can_revert = false;
	bool can_pin = false;
	bool pin_hidden = false;
	bool pinned = false;
	bool use_folding = false;
	bool draw_top_bg = true;
	void _update_popup();
	void _focusable_focused(int p_index);
	bool selectable = true;
	bool selected = false;
	int selected_focusable;
	float split_ratio;
	Vector<Control *> focusables;
	Control *label_reference = nullptr;
	Control *bottom_editor = nullptr;
	PopupMenu *menu = nullptr;
	HashMap<StringName, Variant> cache;
	GDVIRTUAL0(_update_property)