trailing comma in collection causes null pointer exception

[rmorrise]

A poorly formatted JSON document causes the plugin to throw a null pointer exception when validating objects in a collection. This occurs if there is a trailing comma after the last object in the collection. Grails will pass a list containing the objects in the list plus a null object. The plugin does not handle this null object gracefully.

(Reported 07/16/2015 by @hzill)