readerWindow: lock_ui and unlock_ui work before window show
readerWindow: lock_ui and unlock_ui work before window show

We try to lock our ui during a reader search, but when that is
activated from global search, this can happen before the window
is actually shown and we have a backing gdk window.

This makes the lock_ui/unlock_ui functions robust to this case.
[endlessm/eos-sdk#3160]

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib