### Godot version

v4.0 rc6

### System information

Windows 10

### Issue description

When using a ViewportTexture from a SubViewport on a TextureRect, it throws errors after running the scene once and then switching scenes in the editor.
![Pasted image 20230227212626](https://user-images.githubusercontent.com/44676282/221680047-984fa03e-33d1-4856-9a7f-1700b28423cf.png)


### Steps to reproduce

Run the canvas-scene once.
Switch to another scene in the editor.
Switch back to the canvas-scene

### Minimal reproduction project

[ViewportTextureError.zip](https://github.com/godotengine/godot/files/10844099/ViewportTextureError.zip)

ERR_FAIL_COND(p_event.is_null());
	Ref<InputEventMouseButton> mb = p_event;
	if (mb.is_valid()) {
		if (!mb->is_pressed()) {
			gui.mouse_focus_mask.clear_flag(mouse_button_to_mask(mb->get_button_index())); 
		}
	}
}
List<Control *>::Element *Viewport::_gui_add_root_control(Control *p_control) {
	gui.roots_order_dirty = true;
	return gui.roots.push_back(p_control);
}
void Viewport::gui_set_root_order_dirty() {
	ERR_MAIN_THREAD_GUARD;
	gui.roots_order_dirty = true;
}
void Viewport::_gui_force_drag(Control *p_base, const Variant &p_data, Control *p_control) {
	ERR_FAIL_COND_MSG(p_data.get_type() == Variant::NIL, "Drag data must be a value.");
	gui.dragging = true;
	gui.drag_data = p_data;
	gui.mouse_focus = nullptr;
	if (p_control) {
		_gui_set_drag_preview(p_base, p_control);
	}
	_propagate_viewport_notification(this, NOTIFICATION_DRAG_BEGIN);
}
void Viewport::_gui_set_drag_preview(Control *p_base, Control *p_control) {