The last_active_client is set when an AbstractClient gets activated. For
the X11 case the last_active_client was getting reset to nullptr when
the last_active_client gets destroyed. But for the ShellClient that did
not yet happen. This could result in a crash.
This change addresses the problem and adds a test case which triggered
the crash. The condition of the crash are difficult to generate though -
it took me about an hour to write the test for the crash.
- Wayland client must be active
- Explicit focus to null (no active client)
- destroy Wayland window
- X11 client which sets focus on itself without interaction with window manager