use null instead of 'nil' and call trigger_error correctly (more commits to come on that one)

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1300 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9