public void createControl(Composite parent) {
Composite container = new Composite(parent, SWT.NULL);
GridLayout layout = new GridLayout();
container.setLayout(layout);
_treeViewer = createTreeViewer(container);
Tree treeWidget = _treeViewer.getTree();
GridData gridDataTree = new GridData(GridData.FILL_BOTH);
gridDataTree.horizontalSpan = 3;
gridDataTree.widthHint = convertWidthInCharsToPixels(80);
gridDataTree.heightHint = convertHeightInCharsToPixels(20);
treeWidget.setLayoutData(gridDataTree);
treeWidget.addSelectionListener(this);
setControl(container);
performValidation();
}