Bug 191685 [regression]Set default value doesn't work in Report Parameter[0705]
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
previewLabel.setText( convertNullString( previewString ) );
		changeFormat.setEnabled( choiceSet != null );
	}
	private String convertNullString( String str )
	{
		if ( str == null )
		{
			return "";
		}
		return str;
	}
	private boolean containValue( SelectionChoice selectedChoice,
			String newValue, String property )
	{
		for ( Iterator iter = choiceList.iterator( ); iter.hasNext( ); )
		{
			SelectionChoice choice = (SelectionChoice) iter.next( );
			if ( choice != selectedChoice )
			{
				String value = null;
				if ( COLUMN_VALUE.equals( property ) )
				{
					value = choice.getValue( );
					if ( isEqual( value, newValue ) )
					{
						return true;
					}
				}
				if ( COLUMN_DISPLAY_TEXT.equals( property ) )
				{