Bug 79916 [Viewers] isDisposed check needed in AbstractTreeViewer#internalRemove
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java
*           the parent element
     * @param childElements
     *           the child elements to add
     */
    public void add(Object parentElement, Object[] childElements) {
        Assert.isNotNull(parentElement);
        assertElementsNotNull(childElements);
        Widget widget = findItem(parentElement);
        
        if (widget == null)
            return;
        internalAdd(widget, parentElement, childElements);
    }
	/**
     * XXX: temporary API in support of Dirk's explorations
     * into grouping by working sets, which requires viewers
     * to support multiple equal elements.
     */
    protected void internalAdd(Widget widget, Object parentElement, Object[] childElements) {
		Control tree = getControl();
        
        
        if (widget instanceof Item) {
            Item ti = (Item) widget;
            if (!getExpanded(ti)) {
                boolean needDummy = isExpandable(parentElement);
                boolean haveDummy = false;
                
                Item[] items = getItems(ti);
                for (int i = 0; i < items.length; i++) {