### 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()) {
		gui.key_event_accepted = false;
		Point2 mpos = mb->get_position();
		if (mb->is_pressed()) {
			if (!gui.mouse_focus_mask.is_empty()) {
				
				gui.mouse_focus_mask.set_flag(mouse_button_to_mask(mb->get_button_index()));
			} else {
				gui.mouse_focus = gui_find_control(mpos);
				gui.last_mouse_focus = gui.mouse_focus;
				if (!gui.mouse_focus) {
					return;