Bug 151959 inappropriate screen operation
steps to reproduce: - create a report - add a grid item (3x3) - switch to Outline view - expand the grid item to see its rows and cells - select the grid item, and drag it into any of its own cell expected behavior: - is it an invalid screen operation? if it is, it should be banned actual behavior: - the grid item was disappeared surprisingly
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
*            the value list to check
	 * @param toAdd
	 *            the value item to add. This parameter maybe is
	 *            <code>null</code>.
	 * @throws PropertyValueException
	 *             if the structure list or the item to add has any semantic
	 *             error..
	 */
	public void checkSimpleList( Module module, PropertyDefn propDefn,
			List list, Object toAdd ) throws PropertyValueException
	{
		List errorList = SimpleListValidator.getInstance( ).validateForAdding(
				getHandle( module ), propDefn, list, toAdd );
		if ( errorList.size( ) > 0 )
		{
			throw (PropertyValueException) errorList.get( 0 );
		}
	}
	/**
	 * The common logic for both clone and cloneForTemplate.
	 * 
	 * @return the clone element with reference to parent in library
	 * @throws CloneNotSupportedException
	 */
	final private Object baseClone( ) throws CloneNotSupportedException
	{
		DesignElement element = (DesignElement) super.clone( );
		element.container = null;
		element.listeners = null;
		element.derived = null;