### 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)

mb->set_position(pos);
			Control *mouse_focus = gui.mouse_focus;
			
			
			
			if (gui.mouse_focus_mask.is_empty()) {
				gui.mouse_focus = nullptr;
				gui.forced_mouse_focus = false;
			}
			bool stopped = mouse_focus && mouse_focus->can_process() && _gui_call_input(mouse_focus, mb);
			if (stopped) {
				set_input_as_handled();
			}
		}
	}
	Ref<InputEventMouseMotion> mm = p_event;
	if (mm.is_valid()) {
		gui.key_event_accepted = false;
		Point2 mpos = mm->get_position();
		
		if (!gui.drag_attempted && gui.mouse_focus && (mm->get_button_mask().has_flag(MouseButtonMask::LEFT))) {
			gui.drag_accum += mm->get_relative();
			float len = gui.drag_accum.length();
			if (len > 10) {
				{ 
					CanvasItem *ci = gui.mouse_focus;
					while (ci) {
						Control *control = Object::cast_to<Control>(ci);
						if (control) {
							gui.dragging = true;