Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/PropertyMaskHandle.java
* @param index
	 *            the position of this property mask in the list
	 */
	public PropertyMaskHandle( SimpleValueHandle valueHandle, int index )
	{
		super( valueHandle, index );
	}
	/**
	 * Returns the property mask. The possible values are defined in
	 * {org.eclipse.birt.report.model.elements.DesignChoiceConstants}, and they are:
	 * <ul>
	 * <li>PROPERTY_MASK_TYPE_CHANGE
	 * <li>PROPERTY_MASK_TYPE_LOCK
	 * <li>PROPERTY_MASK_TYPE_HIDE
	 * </ul>
	 * 
	 * @return the property mask
	 */
	public String getMask( )
	{
		return getStringProperty( PropertyMask.MASK_MEMBER );
	}
	/**
	 * Sets the property mask. The allowed values are defined in
	 * {org.eclipse.birt.report.model.elements.DesignChoiceConstants}, and they are:
	 * <ul>
	 * <li>PROPERTY_MASK_TYPE_CHANGE
	 * <li>PROPERTY_MASK_TYPE_LOCK
	 * <li>PROPERTY_MASK_TYPE_HIDE
	 * </ul>