Bug 190727 [regression]Default value issue when switch report parameter from textbox to listbox.[0705]
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
sortDirectionChooser.add( CHOICE_ASCENDING );
		sortDirectionChooser.add( CHOICE_DESCENDING );
		sortDirectionChooser.setText( CHOICE_ASCENDING );
		Composite distinctBtnArea = new Composite( sortGroup, SWT.NONE );
		distinctBtnArea.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		distinctBtnArea.setLayout( new GridLayout( ) );
		distinct = new Button( distinctBtnArea, SWT.CHECK );
		distinct.setText( CHECKBOX_DISTINCT );
		distinct.setSelection( true );
	}
	private void clearDefaultValueText( )
	{
		if ( defaultValueChooser == null || defaultValueChooser.isDisposed( ) )
			return;
		String textValue = defaultValueChooser.getText( );
		if ( textValue != null
				&& ( textValue.equals( CHOICE_NULL_VALUE ) || textValue.equals( CHOICE_BLANK_VALUE ) ) )
		{
			defaultValueChooser.setText( "" );
		}
	}
	private void clearDefaultValueChooserSelections( )
	{
		if ( defaultValueChooser == null || defaultValueChooser.isDisposed( ) )
			return;
		if ( defaultValueChooser.getItemCount( ) > 1 )
		{
			defaultValueChooser.remove( CHOICE_NULL_VALUE );
			defaultValueChooser.remove( CHOICE_BLANK_VALUE );
		}