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 dragSetData( DragSourceEvent event )
			{
				if ( TextTransfer.getInstance( )
						.isSupportedType( event.dataType ) )
				{
					TreeItem[] selection = tree.getSelection( );
					if ( selection.length > 0 )
					{
						event.data = selection[0].getData( ITEM_DATA_KEY_TEXT );
					}
				}
			}
		} );
	}
	/**
	 * Insert a text string into the text area
	 * 
	 * @param text
	 */
	protected void insertText( String text )
	{
		StyledText textWidget = expressionViewer.getTextWidget( );
		if ( !textWidget.isEnabled( ) )
		{
			return;
		}
		int selectionStart = textWidget.getSelection( ).x;
		if ( text.equalsIgnoreCase( "x++" ) ) 
		{
			text = textWidget.getSelectionText( ) + "++";