### 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 (gui.key_focus->can_process()) {
				gui.key_focus->_call_gui_input(p_event);
			}
			if (gui.key_event_accepted) {
				set_input_as_handled();
				return;
			}
		}
		Control *from = gui.key_focus ? gui.key_focus : nullptr;
		if (from && p_event->is_pressed()) {
			Control *next = nullptr;
			Ref<InputEventJoypadMotion> joypadmotion_event = p_event;
			if (joypadmotion_event.is_valid()) {
				Input *input = Input::get_singleton();
				if (p_event->is_action_pressed("ui_focus_next") && input->is_action_just_pressed("ui_focus_next")) {
					next = from->find_next_valid_focus();
				}
				if (p_event->is_action_pressed("ui_focus_prev") && input->is_action_just_pressed("ui_focus_prev")) {
					next = from->find_prev_valid_focus();
				}
				if (p_event->is_action_pressed("ui_up") && input->is_action_just_pressed("ui_up")) {
					next = from->_get_focus_neighbor(SIDE_TOP);
				}
				if (p_event->is_action_pressed("ui_left") && input->is_action_just_pressed("ui_left")) {
					next = from->_get_focus_neighbor(SIDE_LEFT);
				}
				if (p_event->is_action_pressed("ui_right") && input->is_action_just_pressed("ui_right")) {
					next = from->_get_focus_neighbor(SIDE_RIGHT);
				}
				if (p_event->is_action_pressed("ui_down") && input->is_action_just_pressed("ui_down")) {