[8u60] Review request for RT-39688: Glass-Mac window close deallocation produces a spurious warning

[Morris Meyer]

David and Kevin,

Please review this very small change to prevent a spurious warning when 
de-allocation occurs for a NSWindow instance after the Java VM has shutdown.

Thanks much,

         --morris

JIRA - https://javafx-jira.kenai.com/browse/RT-39688
WEBREV - http://cr.openjdk.java.net/~morris/RT-39688.01