500 error when character in transfer is deleted

[SpeedyD]

Bug as reported:

(Main branch, last commit parent: da3ebb11)

URL of page: <site>/admin/masterlist/transfers/incoming, <site>/admin/masterlist/trades/incoming, <site>/admin/masterlist/transfers/completed, <site>/admin/masterlist/trades/completed (Likely more?)
Description of bug: When a character has been deleted entirely, if it was ever part of a transfer or trade, whether resolved or not, that trade or transfer will give out a 500 error. I cannot tell if it's in the public area too, but I would presume so.
How was the bug caused (exact steps to reproduce): Add a character to a transfer or trade. Delete the character. Go to the transfer or trade in the admin section.

Further details

...following up #459 and #460. It's the same problem, different location.