### Godot version

v4.2.dev.custom_build [0c2144da9]

### System information

Debian Linux X11 Xfce

### Issue description

Prerequisite: Be in Single Window Mode in the Editor
The Filesystem Dock is unusable when it is floating, which is not expected.
It appears garbled in the top-left corner:
![image](https://github.com/godotengine/godot/assets/6299227/109f7922-97de-44c3-814a-ca7f970e0602)

The following errors are shown:
```
  editor/window_wrapper.cpp:220 - Condition "!is_window_available()" is true.
  editor/window_wrapper.cpp:202 - Condition "!get_window_enabled()" is true. Returning: Rect2i()
  editor/window_wrapper.cpp:207 - Condition "!get_window_enabled()" is true. Returning: -1
  editor/window_wrapper.cpp:207 - Condition "!get_window_enabled()" is true. Returning: -1
```

bisecting lead to #62378
possibly related to #44439

### Steps to reproduce

1. Start a project in the Editor
2. Switch to Single Window Mode in the Editor and reload (if necessary)
3. Make the Filesystem Dock floating

### Minimal reproduction project

Editor issue
List<StringName> tl;
	theme_base->get_theme()->get_icon_list(SNAME("EditorIcons"), &tl);
	for (const StringName &E : tl) {
		if (!ClassDB::class_exists(E)) {
			continue;
		}
		icon_type_cache[E] = theme_base->get_theme()->get_icon(E, SNAME("EditorIcons"));
	}
}
void EditorNode::_enable_pending_addons() {
	for (uint32_t i = 0; i < pending_addons.size(); i++) {
		set_addon_plugin_enabled(pending_addons[i], true);
	}
	pending_addons.clear();
}
void EditorNode::_file_dialog_register(FileDialog *p_dialog) {