### 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
ERR_FAIL_COND_V(!p_object, StringName());
	Ref<Script> scr = p_object->get_script();
	if (scr.is_null() && Object::cast_to<Script>(p_object)) {
		scr = p_object;
	}
	if (scr.is_valid()) {
		Ref<Script> base_scr = scr;
		while (base_scr.is_valid()) {
			StringName name = EditorNode::get_editor_data().script_class_get_name(base_scr->get_path());
			if (name != StringName()) {
				return name;
			}
			
			StringName base = base_scr->get_instance_base_type();
			if (base != StringName() && EditorNode::get_editor_data().get_custom_types().has(base)) {
				const Vector<EditorData::CustomType> &types = EditorNode::get_editor_data().get_custom_types()[base];
				for (int i = 0; i < types.size(); ++i) {
					if (types[i].script == base_scr) {
						return types[i].name;
					}
				}
			}
			base_scr = base_scr->get_base_script();
		}
	}
	return StringName();
}
void EditorNode::_pick_main_scene_custom_action(const String &p_custom_action_name) {