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
dropTarget.setTransfer( new Transfer[]{
			TextTransfer.getInstance( )
		} );
		dropTarget.addDropListener( new DropTargetAdapter( ) {
			public void dragEnter( DropTargetEvent event )
			{
				text.setFocus( );
				if ( event.detail == DND.DROP_DEFAULT )
					event.detail = DND.DROP_COPY;
				if ( event.detail != DND.DROP_COPY )
					event.detail = DND.DROP_NONE;
			}
			public void dragOver( DropTargetEvent event )
			{
				event.feedback = DND.FEEDBACK_SCROLL
						| DND.FEEDBACK_INSERT_BEFORE;
			}
			public void dragOperationChanged( DropTargetEvent event )
			{
				dragEnter( event );
			}
			public void drop( DropTargetEvent event )
			{
				if ( event.data instanceof String )
					insertText( (String) event.data );
			}
		} );
	}
	/**
	 * Disposes resources.