### 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
import_buttons->add_spacer();
	Button *import_add_selected_button = memnew(Button);
	import_add_selected_button->set_text(TTR("Import Selected"));
	import_buttons->add_child(import_add_selected_button);
	import_add_selected_button->connect("pressed", callable_mp(this, &ThemeItemImportTree::_import_selected));
}
void ThemeItemEditorDialog::ok_pressed() {
	if (import_default_theme_items->has_selected_items() || import_editor_theme_items->has_selected_items() || import_other_theme_items->has_selected_items()) {
		confirm_closing_dialog->set_text(TTR("Import Items tab has some items selected. Selection will be lost upon closing this window.\nClose anyway?"));
		confirm_closing_dialog->popup_centered(Size2(380, 120) * EDSCALE);
		return;
	}
	hide();
}
void ThemeItemEditorDialog::_close_dialog() {
	hide();
}
void ThemeItemEditorDialog::_dialog_about_to_show() {
	ERR_FAIL_COND_MSG(edited_theme.is_null(), "Invalid state of the Theme Editor; the Theme resource is missing.");
	_update_edit_types();
	import_default_theme_items->set_edited_theme(edited_theme);
	import_default_theme_items->set_base_theme(ThemeDB::get_singleton()->get_default_theme());
	import_default_theme_items->reset_item_tree();
	import_editor_theme_items->set_edited_theme(edited_theme);
	import_editor_theme_items->set_base_theme(EditorNode::get_singleton()->get_theme_base()->get_theme());
	import_editor_theme_items->reset_item_tree();
	import_other_theme_items->set_edited_theme(edited_theme);
	import_other_theme_items->reset_item_tree();
}
void ThemeItemEditorDialog::_update_edit_types() {