### 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)
RS::get_singleton()->canvas_item_clear(sw.canvas_item);
	Rect2i r = Rect2i(p_window->get_position(), sw.window->get_size());
	if (!p_window->get_flag(Window::FLAG_BORDERLESS)) {
		Ref<StyleBox> panel = p_window->get_theme_stylebox(SNAME("embedded_border"));
		panel->draw(sw.canvas_item, r);
		
		Ref<Font> title_font = p_window->get_theme_font(SNAME("title_font"));
		int font_size = p_window->get_theme_font_size(SNAME("title_font_size"));
		Color title_color = p_window->get_theme_color(SNAME("title_color"));
		int title_height = p_window->get_theme_constant(SNAME("title_height"));
		int close_h_ofs = p_window->get_theme_constant(SNAME("close_h_offset"));
		int close_v_ofs = p_window->get_theme_constant(SNAME("close_v_offset"));
		TextLine title_text = TextLine(p_window->atr(p_window->get_title()), title_font, font_size);
		title_text.set_width(r.size.width - panel->get_minimum_size().x - close_h_ofs);
		title_text.set_direction(p_window->is_layout_rtl() ? TextServer::DIRECTION_RTL : TextServer::DIRECTION_LTR);
		int x = (r.size.width - title_text.get_size().x) / 2;
		int y = (-title_height - title_text.get_size().y) / 2;
		Color font_outline_color = p_window->get_theme_color(SNAME("title_outline_modulate"));
		int outline_size = p_window->get_theme_constant(SNAME("title_outline_size"));
		if (outline_size > 0 && font_outline_color.a > 0) {
			title_text.draw_outline(sw.canvas_item, r.position + Point2(x, y), outline_size, font_outline_color);
		}
		title_text.draw(sw.canvas_item, r.position + Point2(x, y), title_color);
		bool pressed = gui.subwindow_focused == sw.window && gui.subwindow_drag == SUB_WINDOW_DRAG_CLOSE && gui.subwindow_drag_close_inside;
		Ref<Texture2D> close_icon = p_window->get_theme_icon(pressed ? "close_pressed" : "close");
		close_icon->draw(sw.canvas_item, r.position + Vector2(r.size.width - close_h_ofs, -close_v_ofs));
	}
	RS::get_singleton()->canvas_item_add_texture_rect(sw.canvas_item, r, sw.window->get_texture()->get_rid());
}
void Viewport::_sub_window_grab_focus(Window *p_window) {