I am getting a warning above (IN_DEV) in applications/cms/sources/Categories/Categories.php line 1615

		foreach( $this->children() as $child )
		{
			$childLastPost = $child->lastPost();

			if( !$result OR $childLastPost['date'] > $result['date'] )
			{
				$result = $childLastPost;
			}
		}

You do not check if $childLastPost is NULL, before trying to get $childLastPost['date']. 

Thank you for bringing this issue to our attention! I can confirm this should be further reviewed and I have logged an internal bug report for our development team to investigate and address as necessary, in a future maintenance release.