Fixes a bug in Tree where a null pointer exception can occur if a click event propagates to the root TreeItem, which can happen in IE6.  We now check that the target item is not the root TreeItem.

Patch by: jlabanca
Review by: TBR (rajeev)
 


git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.5@3391 8db76d5a-ed1c-0410-87a9-c151d255dfc7