Bug 124600 delete Grid Column causes NPE thrown out in error log
steps to reproduce: - insert a 3*3 grid - delete grid columns one by one expected behavior: - these operations should completed without any error. actual behavior: - after i deleted the 3rd column, NPE was thrown out in error log.
UI/org.eclipse.birt.report.designer.core/src/org/eclipse/birt/report/designer/util/DNDUtil.java
*         the last position in the container
	 */
	public static int calculateNextPosition( Object targetObj, int canContain )
	{
		int position = -1;
		if ( canContain == CONTAIN_PARENT )
		{
			DesignElementHandle afterHandle = getDesignElementHandle( targetObj );
			if ( afterHandle != null )
			{
				position = afterHandle.getContainerSlotHandle( ).findPosn(
						afterHandle );
				position++;
			}
		}
		return position;
	}
	/**
	 * Adds new object to container
	 * 
	 * @param container
	 *            container, not null
	 * @param handle
	 *            new object. If new object is null, create nothing
	 * @throws SemanticException
	 */
	public static void addElementHandle( Object container,
			DesignElementHandle handle ) throws SemanticException
	{
		assert ( container != null );