DOMImplStandard.java:125 Uncaught TypeError: Cannot read property 'contains' of nullisOrHasChild_1_g$ @ DOMImplStandard.java:125$isOrHasChild_0_g$ @ Node.java:280onBrowserEvent_17_g$ @ ActionCell.java:87fireEventToCell_0_g$ @ CellList.java:360onBrowserEvent2_1_g$ @ CellList.java:453onBrowserEvent_3_g$ @ AbstractHasData.java:738dispatchEventImpl_0_g$ @ DOM.java:1480dispatchEvent_4_g$ @ DOM.java:1419dispatchEvent_6_g$ @ DOMImplStandard.java:317apply_0_g$ @ Impl.java:247entry0_0_g$ @ Impl.java:306(anonymous function) @ Impl.java:72


The idea is to add a button inside a cell to delete that cell. The cell and the button should do 2 different things, and it works right now as expected but it keeps giving this nullpointer. Also the button should be visible only when the cell is selected.
I created my own ActionCell, a Delegate for the button click, SingleSelectionModel for the CellList.
class MyActionCell<T> extends ActionCell<String> {
    static String selectedSelf = "";
    public MyActionCell(String text,
            com.google.gwt.cell.client.ActionCell.Delegate<String> delegate) {
        super(text, delegate);
        // TODO Auto-generated constructor stub
    }

    @Override
    public void render(Context context, String value, SafeHtmlBuilder sb) {
        if(value==null)return;
        Window.alert("Rendering: " +value);
        SafeHtml title = new SafeHtmlBuilder().appendEscaped(value)
                .toSafeHtml();
        sb.append(title);
        if (selectedSelf.contentEquals(value)) {
            SafeHtml html = new SafeHtmlBuilder()
                    .appendHtmlConstant(
                            "<button type=\"button\" tabindex=\"-1\">")
                    .appendEscaped(value)
                    .appendHtmlConstant("</button>").toSafeHtml();
            sb.append(html);
        }

    }

    public static void setSelected(String selected){
        selectedSelf = selected;
    }
}

    private final Delegate<String> delegate = new Delegate<String>(){

        @Override
        public void execute(String clickedPack) {
            if(clickedPack!=null){
            Window.alert("CLicked: "
                    +clickedPack);
            }
        }

    };
MyActionCell<String> ac = new MyActionCell<String>("name",delegate);
    final SingleSelectionModel<String> selectionModel = new SingleSelectionModel<String>();

selectionModel
        .addSelectionChangeHandler(new SelectionChangeEvent.Handler() {
            public void onSelectionChange(
                    SelectionChangeEvent event) {
                String selected = selectionModel
                        .getSelectedObject();

                Window.alert(selected);
                if (selected != null) {
                    //selectionModel.getSelectedObject().setSelected(selected);
                    MyActionCell.setSelected(selected);
                    selectedPackage = selected;
                    updatePackageHolderMiddle();
                }
            }
        });