RFR: JDK-8221987: NPE in javafx.graphics/javafx.stage.Window$TKBoundsConfigurator.apply

[Christian Heilmann]

Please review the fix for "NPE in 
javafx.graphics/javafx.stage.Window$TKBoundsConfigurator.apply":
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8221987
https://github.com/javafxports/openjdk-jfx/pull/431