Return type of Main.layoutManager.getWorkAreaForMonitor is incorrect.

[KSXGitHub]

It is currently void.

[schnz]

Published a fix under @girs/gnome-shell@45.0.0-beta9.

Alternatively, it is also possible to directly use the Workspace Manager without using the (manually typed) gnome-shell package.

import Shell from "gi://Shell";

Shell.Global.get().workspace_manager.get_active_workspace().get_work_area_for_monitor(index)