[TableView] NestedTableColumnHeader crashes in dispose() during layout

Provoked by rearranging columns on a multi-column grid with nested columns.

Construct a window from the following .fxml:

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<AnchorPane prefHeight="480.0" prefWidth="600.0" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8"
            fx:controller="sample.Controller">
    <children>
        <TableView layoutX="32.0" layoutY="-13.0" prefHeight="200.0" prefWidth="200.0" AnchorPane.bottomAnchor="0.0"
                   AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0"
                   xmlns:fx="http://javafx.com/fxml">
            <columns>
                <TableColumn prefWidth="75.0" text="C1"/>
                <TableColumn fx:id="middlecolumn" prefWidth="75.0" text="C2">
                    <columns>
                        <TableColumn prefWidth="75.0" text="C21"/>
                        <TableColumn prefWidth="75.0" text="C22"/>
                        <TableColumn prefWidth="75.0" text="C23"/>
                    </columns>
                </TableColumn>
                <TableColumn prefWidth="75.0" text="C3"/>
            </columns>
        </TableView>
    </children>
</AnchorPane>

Drag the right hand most column and drop it in the table.
See the crash.

dispose() is getting a null reference out of dragRects.get():

        for (int i = 0; i < dragRects.size(); i++) {
            Rectangle rect = dragRects.get(i);
            rect.visibleProperty().unbind();
        }

causing a stack trace in the rect.visibleProperty().unbind() call
This appears to be caused by declaring dragRects() as a Map but treating it as a List.
Should be fixable by using iteration over the map ...

Stack trace:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at com.sun.javafx.scene.control.skin.NestedTableColumnHeader.dispose(NestedTableColumnHeader.java:323)
at com.sun.javafx.scene.control.skin.NestedTableColumnHeader.updateTableColumnHeaders(NestedTableColumnHeader.java:265)
at com.sun.javafx.scene.control.skin.NestedTableColumnHeader.checkState(NestedTableColumnHeader.java:519)
at com.sun.javafx.scene.control.skin.NestedTableColumnHeader.computePrefHeight(NestedTableColumnHeader.java:401)
at javafx.scene.Parent.prefHeight(Parent.java:918)
at javafx.scene.layout.Region.prefHeight(Region.java:1438)
at com.sun.javafx.scene.control.skin.TableHeaderRow.computePrefHeight(TableHeaderRow.java:344)
at com.sun.javafx.scene.control.skin.TableHeaderRow.computeMinHeight(TableHeaderRow.java:339)
at javafx.scene.Parent.minHeight(Parent.java:946)
at javafx.scene.layout.Region.minHeight(Region.java:1404)
at javafx.scene.control.SkinBase.computeMinHeight(SkinBase.java:254)
at javafx.scene.control.Control.computeMinHeight(Control.java:485)
at javafx.scene.Parent.minHeight(Parent.java:946)
at javafx.scene.layout.Region.minHeight(Region.java:1404)
at javafx.scene.layout.AnchorPane.computeHeight(AnchorPane.java:297)
at javafx.scene.layout.AnchorPane.computeMinHeight(AnchorPane.java:246)
at javafx.scene.Parent.minHeight(Parent.java:946)
at javafx.scene.layout.Region.minHeight(Region.java:1404)
at javafx.scene.layout.Region.computeChildMinAreaHeight(Region.java:1700)
at javafx.scene.layout.BorderPane.getAreaHeight(BorderPane.java:618)
at javafx.scene.layout.BorderPane.computeMinHeight(BorderPane.java:415)
at javafx.scene.Parent.minHeight(Parent.java:946)
at javafx.scene.layout.Region.minHeight(Region.java:1404)
at javafx.scene.layout.BorderPane.layoutChildren(BorderPane.java:485)
at javafx.scene.Parent.layout(Parent.java:1076)
at javafx.scene.Parent.layout(Parent.java:1082)
at javafx.scene.Parent.layout(Parent.java:1082)
at javafx.scene.Scene.doLayoutPass(Scene.java:568)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2367)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:330)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:479)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:460)
at com.sun.javafx.tk.quantum.QuantumToolkit$13.run(QuantumToolkit.java:327)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
at java.lang.Thread.run(Thread.java:744)