Regression - ComboBoxTableCell: throws NPE when clicking on arrow

To reproduce, use example in https://bugs.openjdk.java.net/browse/JDK-8138683

- start editing such that the popup is open
- click on the arrow
- expected: popup closes (nothing else)
- actual: throws NPE

First lines of the stacktrace:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at javafx.scene.control.skin.ComboBoxPopupControl.positionAndShowPopup(ComboBoxPopupControl.java:411)
at javafx.scene.control.skin.ComboBoxPopupControl.show(ComboBoxPopupControl.java:249)
at javafx.scene.control.skin.ComboBoxBaseSkin.lambda$new$906(ComboBoxBaseSkin.java:127)
at com.sun.javafx.scene.control.LambdaMultiplePropertyChangeListenerHandler.lambda$new$10(LambdaMultiplePropertyChangeListenerHandler.java:50)
at javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:89)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:360)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.ReadOnlyBooleanPropertyBase.fireValueChangedEvent(ReadOnlyBooleanPropertyBase.java:72)
at javafx.beans.property.ReadOnlyBooleanWrapper.fireValueChangedEvent(ReadOnlyBooleanWrapper.java:103)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:111)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:145)
at javafx.scene.control.ComboBoxBase.setShowing(ComboBoxBase.java:185)
at javafx.scene.control.ComboBoxBase.show(ComboBoxBase.java:391)
at com.sun.javafx.scene.control.behavior.ComboBoxBaseBehavior.show(ComboBoxBaseBehavior.java:279)

Worked as expected in release 8u60, could be introduced by https://bugs.openjdk.java.net/browse/JDK-8138683