Unfortunately this is a known bug with deluge. Here's the steps which produce the error:
1. Add a magnet link / torrent which doesn't download any data
2. Try to remove the torrent before any data has been downloaded
It has already been reported to deluge here:
https://dev.deluge-torrent.org/ticket/3325
https://forum.deluge-torrent.org/viewtopic.php?t=55472
We believe this will be addressed in the next release, however we're not sure when that will be. Until then you may have to leave the torrents in deluge until this is fixed, and avoid downloading magnet links which have no peers/seeds.
Sorry that we cannot be of more help.
