For the moment I haven't found a way to deterministically reproduce this bug.

java.lang.NullPointerException
at com.sun.javafx.css.StyleManager.findMatchingStyles(StyleManager.java:1772)
at javafx.scene.CssStyleHelper.createStyleHelper(CssStyleHelper.java:111)
at javafx.scene.Node.reapplyCss(Node.java:8962)
at javafx.scene.Node.impl_reapplyCSS(Node.java:8925)
at javafx.scene.Node.invalidatedScenes(Node.java:833)
at javafx.scene.Node.setScenes(Node.java:898)
at javafx.scene.Parent.scenesChanged(Parent.java:656)
at javafx.scene.Node.invalidatedScenes(Node.java:832)
at javafx.scene.Node.setScenes(Node.java:898)
at javafx.scene.Parent.scenesChanged(Parent.java:656)
at javafx.scene.Node.invalidatedScenes(Node.java:832)
at javafx.scene.Node.setScenes(Node.java:898)
at javafx.scene.Parent.scenesChanged(Parent.java:656)
at javafx.scene.Node.invalidatedScenes(Node.java:832)
at javafx.scene.Node.setScenes(Node.java:898)
at javafx.scene.Parent.scenesChanged(Parent.java:656)
at javafx.scene.Node.invalidatedScenes(Node.java:832)
at javafx.scene.Node.setScenes(Node.java:898)
at javafx.scene.Parent$1.onChanged(Parent.java:269)
at com.sun.javafx.collections.TrackableObservableList.lambda$new$19(TrackableObservableList.java:45)
at com.sun.javafx.collections.TrackableObservableList$$Lambda$61/2053650390.onChanged(Unknown Source)
at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:329)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:73)
at javafx.collections.ObservableListBase.fireChange(ObservableListBase.java:233)
at javafx.collections.ListChangeBuilder.commit(ListChangeBuilder.java:482)
at javafx.collections.ListChangeBuilder.endChange(ListChangeBuilder.java:541)
at javafx.collections.ObservableListBase.endChange(ObservableListBase.java:205)
at javafx.collections.ModifiableObservableListBase.add(ModifiableObservableListBase.java:155)
at java.util.AbstractList.add(AbstractList.java:108)
at com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:209)