Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
{
			throw new SemanticError( element,
					SemanticError.DESIGN_EXCEPTION_INVALID_ELEMENT_REF );
		}
		return result;
	}
	/**
	 * The property is a simple value list. If property is a list property, the
	 * method will check to see if the current element has the local list value,
	 * if it has, the method returns, otherwise, a copy of the list value
	 * inherited from container or parent will be set locally on the element
	 * itself.
	 * <p>
	 * This method is supposed to be used when we need to change the value of a
	 * composite property( a simple list property ). These kind of property is
	 * inherited as a whole, so when the value changed from a child element.
	 * This method will be called to ensure that a local copy will be made, so
	 * change to the child won't affect the original value in the parent.
	 * 
	 * @param ref
	 *            a reference to a list property or member.
	 */
	void makeLocalCompositeValue( ElementPropertyDefn prop )
	{
		assert prop != null;
		
		ArrayList list = (ArrayList) element.getLocalProperty( module, prop );
		if ( list != null )
			return;