Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/SortKeyHandle.java
*/
	public SortKeyHandle( SimpleValueHandle valueHandle, int index )
	{
		super( valueHandle, index );
	}
	/**
	 * Returns an expression that gives the sort key on which to sort. The
	 * simplest case is the name of a column. The expression can also be an
	 * expression that includes columns. When used for a group, the expression
	 * can contain an aggregate computed over the group.
	 * 
	 * @return the key to sort
	 * 
	 * @see #setKey(String)
	 */
	public String getKey( )
	{
		return getStringProperty( SortKey.KEY_MEMBER );
	}
	/**
	 * Sets an expression that gives the sort key on which to sort.
	 * 
	 * @param key
	 *            the key to sort
	 * 
	 * @see #getKey()
	 */
	public void setKey( String key )
	{
		setPropertySilently( SortKey.KEY_MEMBER, key );