RFR: JDK-8226274: NPE in WinWindow.notifyMoving when Stage with no Scene is shown on 2nd monitor

[Kevin Rushforth]

Please review the following fix for an NPE on a multi-monitor system:

https://bugs.openjdk.java.net/browse/JDK-8226274
https://github.com/javafxports/openjdk-jfx/pull/503

-- Kevin