Null Pointer exception inside javafx layout

An NPE happens inside javafx code (can't make anything out of it):

java.lang.NullPointerException
at javafx.scene.Parent.impl_resizeChildren(Parent.java:936)
at javafx.scene.Group.layoutChildren(Group.java:139)
at javafx.scene.Parent.layout(Parent.java:900)
at javafx.scene.Parent.layout(Parent.java:910)
at javafx.scene.Parent.layout(Parent.java:910)
at javafx.scene.Parent.layout(Parent.java:910)
at javafx.scene.Scene.layoutDirtyRoots(Scene.java:451)
at javafx.scene.Scene.doLayoutPass(Scene.java:424)
at javafx.scene.Scene.access$2800(Scene.java:132)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:1750)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:292)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:398)
at com.sun.javafx.tk.quantum.QuantumToolkit$10.run(QuantumToolkit.java:330)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
at com.sun.glass.ui.win.WinApplication$2$1.run(WinApplication.java:62)
at java.lang.Thread.run(Thread.java:662)
java.lang.NullPointerException
at javafx.scene.Parent.impl_computeGeomBounds(Parent.java:1092)
at javafx.scene.Node.getGeomBounds(Node.java:2785)
at javafx.scene.Node.getLocalBounds(Node.java:2751)
at javafx.scene.Node.getTransformedBounds(Node.java:2700)
at javafx.scene.Parent.impl_computeGeomBounds(Parent.java:1093)
at javafx.scene.Node.getGeomBounds(Node.java:2785)
at javafx.scene.Node.getLocalBounds(Node.java:2751)
at javafx.scene.Node.getTransformedBounds(Node.java:2685)
at javafx.scene.Parent.impl_computeGeomBounds(Parent.java:1093)
at javafx.scene.Node.getGeomBounds(Node.java:2785)
at javafx.scene.Node.getLocalBounds(Node.java:2751)
at javafx.scene.Node.getTransformedBounds(Node.java:2685)
at javafx.scene.Parent.impl_computeGeomBounds(Parent.java:1093)
at javafx.scene.Node.getGeomBounds(Node.java:2785)
at javafx.scene.Node.getLocalBounds(Node.java:2751)
at javafx.scene.Node.updateTxBounds(Node.java:2822)
at javafx.scene.Node.getTransformedBounds(Node.java:2681)
at javafx.scene.Node.impl_updatePG(Node.java:466)
at javafx.scene.Parent.impl_updatePG(Parent.java:85)
at javafx.scene.Node.impl_syncPGNode(Node.java:429)
at javafx.scene.Scene$ScenePulseListener.synchronizeSceneNodes(Scene.java:1657)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:1757)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:292)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:398)
at com.sun.javafx.tk.quantum.QuantumToolkit$10.run(QuantumToolkit.java:330)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
at com.sun.glass.ui.win.WinApplication$2$1.run(WinApplication.java:62)
at java.lang.Thread.run(Thread.java:662)