### 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)

if (get_base_window() == p_window) {
		gui_release_focus();
	}
}
bool Viewport::_gui_control_has_focus(const Control *p_control) {
	return gui.key_focus == p_control;
}
void Viewport::_gui_control_grab_focus(Control *p_control) {
	if (gui.key_focus && gui.key_focus == p_control) {
		
		return;
	}
	get_tree()->call_group("_viewports", "_gui_remove_focus_for_window", (Node *)get_base_window());
	if (p_control->is_inside_tree() && p_control->get_viewport() == this) {
		gui.key_focus = p_control;
		emit_signal(SNAME("gui_focus_changed"), p_control);
		p_control->notification(Control::NOTIFICATION_FOCUS_ENTER);
		p_control->queue_redraw();
	}
}
void Viewport::_gui_accept_event() {