### Godot version

v4.0.beta.custom_build [0a9e6e478]

### System information

Ubuntu, Nvidia, Vulkan

### Issue description

When a Camera2D is freed - either explicitly or by being a child of a scene that is being switched away from via a method like `get_tree().change_scene_to_file()` - then this camera somehow stays active instead of Godot properly switching to no-camera-mode.

`get_viewport().get_camera_2d()` then returns all sorts of funny things from `<Freed Object>`  (it's supposed to return `null`) to arbitrary nodes of the current scene (not cameras).

This can lead to crashes.

### Steps to reproduce

checkout the MRP.

note that the "Go to scene 2" button switches places after pressing it.
that is because a camera is still active that does not exist in the current tree.

restart it a couple of times and spam the "Go to scene 2" button.
notice the garbage output.

### Minimal reproduction project

[godot_4_camera_bug.zip](https://github.com/godotengine/godot/files/10560455/godot_4_camera_bug.zip)
next = from->_get_focus_neighbor(SIDE_BOTTOM);
				}
			} else {
				if (p_event->is_action_pressed("ui_focus_next", true, true)) {
					next = from->find_next_valid_focus();
				}
				if (p_event->is_action_pressed("ui_focus_prev", true, true)) {
					next = from->find_prev_valid_focus();
				}
				if (p_event->is_action_pressed("ui_up", true, true)) {
					next = from->_get_focus_neighbor(SIDE_TOP);
				}
				if (p_event->is_action_pressed("ui_left", true, true)) {
					next = from->_get_focus_neighbor(SIDE_LEFT);
				}
				if (p_event->is_action_pressed("ui_right", true, true)) {
					next = from->_get_focus_neighbor(SIDE_RIGHT);
				}
				if (p_event->is_action_pressed("ui_down", true, true)) {
					next = from->_get_focus_neighbor(SIDE_BOTTOM);
				}
			}
			if (next) {
				next->grab_focus();
				set_input_as_handled();
			}
		}
	}
}
void Viewport::_perform_drop(Control *p_control, Point2 p_pos) {