### 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_MAIN_THREAD_GUARD;
	if (gui.subwindow_focused) {
		gui.subwindow_focused->push_text_input(p_text);
		return;
	}
	if (gui.key_focus) {
		gui.key_focus->call("set_text", p_text);
	}
}
Viewport::SubWindowResize Viewport::_sub_window_get_resize_margin(Window *p_subwindow, const Point2 &p_point) {
	if (p_subwindow->get_flag(Window::FLAG_BORDERLESS) || p_subwindow->get_flag(Window::FLAG_RESIZE_DISABLED)) {
		return SUB_WINDOW_RESIZE_DISABLED;
	}
	Rect2i r = Rect2i(p_subwindow->get_position(), p_subwindow->get_size());
	int title_height = p_subwindow->get_theme_constant(SNAME("title_height"));
	r.position.y -= title_height;
	r.size.y += title_height;
	if (r.has_point(p_point)) {
		return SUB_WINDOW_RESIZE_DISABLED; 
	}
	int dist_x = p_point.x < r.position.x ? (p_point.x - r.position.x) : (p_point.x > (r.position.x + r.size.x) ? (p_point.x - (r.position.x + r.size.x)) : 0);
	int dist_y = p_point.y < r.position.y ? (p_point.y - r.position.y) : (p_point.y > (r.position.y + r.size.y) ? (p_point.y - (r.position.y + r.size.y)) : 0);
	int limit = p_subwindow->get_theme_constant(SNAME("resize_margin"));
	if (ABS(dist_x) > limit) {
		return SUB_WINDOW_RESIZE_DISABLED;
	}