ui: exclude a findbugs warning in AsyncQuery

Warning of type UWF_NULL_FIELD was produced on converterCallback
field, though it is initialized by other classes, hence it's a false
warning.

Change-Id: I562f96123aea747c76e37fabde0a51fdd36dbbbb
Signed-off-by: Alissa Bonas <abonas@redhat.com>