Bug 147687 Assert err happened with design ui
Run BIRT in workbench with argument -ea. Assert err happened when using design ui to open a design file.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/internal/ui/dialogs/ExpressionTreeSupport.java
* 
	 */
	public void dispose( )
	{
		if ( dropTarget != null && !dropTarget.isDisposed( ) )
		{
			dropTarget.dispose( );
		}
	}
	/**
	 * Sets the tree model.
	 * 
	 * @param tree
	 */
	public void setTree( Tree tree )
	{
		this.tree = tree;
	}
	protected Tree getTree( )
	{
		return tree;
	}
	/**
	 * Sets the viewer to use.
	 * 
	 * @param expressionViewer
	 */
	public void setExpressionViewer( SourceViewer expressionViewer )
	{
		this.expressionViewer = expressionViewer;