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
{
		for ( int i = 0; i < texts.length; i++ )
		{
			createSubTreeItem( parent,
					texts[i][0],
					image,
					texts[i][0],
					texts[i][1],
					true );
		}
	}
	/**
	 * Adds mouse track listener.Must set Tree before execution.
	 * 
	 */
	public void addMouseTrackListener( )
	{
		Assert.isNotNull( tree );
		tree.addMouseTrackListener( new MouseTrackAdapter( ) {
			public void mouseHover( MouseEvent event )
			{
				Widget widget = event.widget;
				if ( widget == tree )
				{
					Point pt = new Point( event.x, event.y );
					TreeItem item = tree.getItem( pt );
					if ( item == null )
						tree.setToolTipText( "" );
					else
					{