NPE in GridPane when removing and adding ColumnConstraints

To reproduce:
1) unzip the attached project
2) setup jfxrt.jar, javafx-beans-dt.jar, javafx-designtime.jar
3) Build and run
4) Click "Add Grid Pane" button
5) Click "Delete Column 1" button
6) Click "Insert Column At 1" button
=> the exception below is thrown:


java.lang.NullPointerException
at javafx.scene.layout.GridPane.computeColumnMetrics(GridPane.java:1267)
at javafx.scene.layout.GridPane.computeGridMetrics(GridPane.java:1078)
at javafx.scene.layout.GridPane.layoutChildren(GridPane.java:1414)
at javafx.scene.Parent.layout(Parent.java:912)
at javafx.scene.Parent.layout(Parent.java:922)
at javafx.scene.Parent.layout(Parent.java:922)
at javafx.scene.Scene.layoutDirtyRoots(Scene.java:474)
at javafx.scene.Scene.doLayoutPass(Scene.java:447)
at javafx.scene.Scene.access$3000(Scene.java:133)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:1851)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:332)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:459)
at com.sun.javafx.tk.quantum.QuantumToolkit$8.run(QuantumToolkit.java:326)