Added documentation to HTMLTable.addTableListener() to explain that getCellForEvent() can return null.  We also check for null in the mail sample.

Patch by: jlabanca
Review by: ecc (desk)



git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4625 8db76d5a-ed1c-0410-87a9-c151d255dfc7