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
protected static int handleValidateTargetCanContainByContainer(
			Object targetObj, Object[] childHandles, boolean validateContainer )
	{
		if ( childHandles.length == 0 )
		{
			return CONTAIN_NO;
		}
		for ( int i = 0; i < childHandles.length; i++ )
		{
			if ( !( childHandles[i] instanceof DesignElementHandle )
					|| handleValidateTargetCanContainByContainer( targetObj,
							(DesignElementHandle) childHandles[i],
							validateContainer ) == CONTAIN_NO )
			{
				return CONTAIN_NO;
			}
		}
		return CONTAIN_THIS;
	}
	protected static int handleValidateTargetCanContainElementHandle(
			DesignElementHandle targetHandle, DesignElementHandle childHandle,
			boolean validateContainer )
	{
		if ( targetHandle instanceof CascadingParameterGroupHandle )
		{
			return childHandle.getContainer( ) == targetHandle
					? CONTAIN_THIS
					: CONTAIN_NO;
		}
		else if ( targetHandle.canContain( DEUtil