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
* @param targetObj
	 *            target elements
	 * @param dragObjType
	 *            specified type of transfer data. Type should get from
	 *            <code>ReportDesignConstants</code>
	 * @see ReportDesignConstants
	 * @return if target elements can be dropped
	 */
	public static boolean handleValidateTargetCanContainType( Object targetObj,
			String dragObjType )
	{
		DesignElementHandle targetHandle = null;
		int slotId = 0;
		targetObj = unwrapToModel( targetObj );
		if ( targetObj instanceof DesignElementHandle )
		{
			targetHandle = (DesignElementHandle) targetObj;
			slotId = DEUtil.getDefaultSlotID( targetObj );
		}
		else if ( targetObj instanceof SlotHandle )
		{
			targetHandle = ( (SlotHandle) targetObj ).getElementHandle( );
			slotId = ( (SlotHandle) targetObj ).getSlotID( );
		}
		else
			return false;
		return targetHandle.canContain( slotId, dragObjType );
	}
	/**
	 * Returns the table or grid parent