Bug 153405 Null parameter value is inconsistent between designer and preview
Description: Null parameter value is inconsistent between designer and preview Steps to reproduce: 1. New a report parameter, list-box, tick AllowNull. 2. Click New button, click OK without input anything. 3. Drag the parameter to Layout and preview Actual result: After step2, display text is "Null" and value is empty. However, when preview, the display text is "Null Value". Expected result: Display text and value should be consistent between Edit Parameter Dialog in Layout and Parameter dialog during preview.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
}
		} );
		
		createLabel( composite, LABEL_SELECT_DISPLAY_TEXT );
		
		
		displayTextChooser = new Combo( composite, SWT.BORDER | SWT.DROP_DOWN );
		displayTextChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		Button displayTextExprButton = new Button( composite, SWT.PUSH );
		displayTextExprButton.setText( "..." ); 
		displayTextExprButton.setToolTipText( Messages.getString( "ParameterDialog.toolTipText.OpenExprButton" ) );
		displayTextExprButton.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent event )
			{
				ExpressionBuilder expressionBuilder = new ExpressionBuilder( getExpression( displayTextChooser.getText( ) ) );
				expressionBuilder.setExpressionProvier( new ParameterExpressionProvider( inputParameter,
						dataSetChooser.getText( ) ) );
				if ( expressionBuilder.open( ) == OK )
				{
					setExpression( displayTextChooser,
							expressionBuilder.getResult( ).trim( ) );
				}
			}
		} );
		
		createDefaultEditor( );
		listLimit.setEditable( true );
	}
	private void createDefaultEditor( )
	{