Bug 182673 No default value error message prompts incorrectly
Steps to reproduce: 1. New a report parameter; 2. Choose control type as Combo; 3. Choose parameter as dynamic type; Expected result: As the isRequired is false by default, parameter's default value can be null value. There should be no error message about no default value prompt. Actual result; Error messages prompts
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
/**
	 * Gets the internal name of the control type from the display name
	 */
	private String getSelectedControlType( )
	{
		String displayText = controlTypeChooser.getText( );
		if ( StringUtil.isBlank( displayText ) )
		{
			return getInputControlType( );
		}
		if ( DISPLAY_NAME_CONTROL_COMBO.equals( displayText ) )
		{
			return PARAM_CONTROL_COMBO;
		}
		if ( DISPLAY_NAME_CONTROL_LIST.equals( displayText ) )
		{
			return PARAM_CONTROL_LIST;
		}
		return CONTROL_TYPE_CHOICE_SET.findChoiceByDisplayName( displayText )
				.getName( );
	}
	private void changeDataType( )
	{
		String type = getSelectedDataType( );
		if ( type.equals( lastDataType ) )
		{
			return;
		}
		if ( buildControlTypeList( type ) )
		{