fix: Exception when cosmicDock.stateObj.dockManager may be undefined

[mmstick]

Issue was reported here https://chat.pop-os.org/pop-os/pl/ttud6b16rt8qxrsz9w16xf38rc

There's a possibility that the dockManager field in the cosmic dock extension may be undefined. We should check if the field exists to prevent an exception from breaking things in cosmic.