#18 removed NPE from ResourceWindowSearch

An NPE occured because if no topic is selected, the topic is null and
getName() of the topic was called anyway. A check for null wass added.
 #18 disable the search button if no category is selected

In ResourceWindowSearch the search button should be disabled, if no
category is selected.
Merge branch 'bug/18-NPE-in-ResourceWindowSearch' into dev