### Godot version

v4.0.beta.custom_build [eaf306e0b]

### System information

Windows 10

### Issue description

Selecting a `Window` node in the Editor's SceneTree results in the message
```
editor/editor_data.cpp:1058 - Parameter "p_node" is null.
```
and the Inspector shows empty content instead of the Window's properties.

I would expect, that clicking on a node in the SceneTree lets me edit the properties of the node.

Bisecting shows that this behavior got introduced in #71770

### Steps to reproduce

1. Create a Window node
2. Click on the Window node in the Editor's SceneTree

### Minimal reproduction project

N/A
EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
	ur->create_action(TTR("Unpin Stylebox"));
	ur->add_do_method(this, "_unpin_leading_stylebox");
	ur->add_undo_method(this, "_pin_leading_stylebox", leading_stylebox.item_name, leading_stylebox.stylebox);
	ur->commit_action();
}
void ThemeTypeEditor::_unpin_leading_stylebox() {
	if (leading_stylebox.stylebox.is_valid()) {
		leading_stylebox.stylebox->disconnect("changed", callable_mp(this, &ThemeTypeEditor::_update_stylebox_from_leading));
	}
	LeadingStylebox leader;
	leader.pinned = false;
	leading_stylebox = leader;
	_update_type_items();
}
void ThemeTypeEditor::_update_stylebox_from_leading() {