[8u60] Request for review: RT-40385: [Mac] NPE in window close unit tests

[Morris Meyer]

David and Kevin,

Please review my fix for the recent regression caused by my fix for 
RT-39813.  Both changes crop up in the systemTests and are necessary to 
adjust for the close sequence timing change introduced in RT-39813.

Tested using:

gradle -PFULL_TEST=true :systemTests:test

Thanks much.

        --morris

WEBREV - http://cr.openjdk.java.net/~morris/RT-40385.01/
JIRA - https://javafx-jira.kenai.com/browse/RT-40385