Fixed a bug in HandlerManager where removing the last handler of a given EventType twice would result in an NPE because we assume that the list of event handlers is not null.  Now, this results in an AssertionError.

Patch by: jlabanca
Review by: rjrjr



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5266 8db76d5a-ed1c-0410-87a9-c151d255dfc7