private void initializeDragAndDrop() {
graphView.setDragEnabled(true);
mxGraphControl graphControl = graphView.getGraphControl();
// graphControl.setDropTarget(graphControl.getDropTarget());
dndHandler = new ViewDragAndDropHandler(graphControl, this, DnDConstants.ACTION_MOVE) {
/**
* checks if a drag is possible
* <ul>
* <li>the cell is not null</li>