### 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_MSG(p_class.is_empty(), nullptr, "Class name cannot be empty.");
	EditorData &ed = EditorNode::get_editor_data();
	
	if (p_script.is_valid()) {
		Ref<Texture2D> script_icon = ed.get_script_icon(p_script);
		if (script_icon.is_valid()) {
			return script_icon;
		}
	}
	
	
	
	Ref<Texture2D> ext_icon = ed.extension_class_get_icon(p_class);
	if (ext_icon.is_valid()) {
		return ext_icon;
	}
	
	
	const EditorData::CustomType *ctype = ed.get_custom_type_by_name(p_class);
	if (ctype && ctype->icon.is_valid()) {
		return ctype->icon;