Wekan : 0.50 (but I think this problem exists since the rest api for deleting user exists)
Description : after a user is deleted by rest api, if that user was affected to a board, the cards in this board can't be opened if that card have a description.

How to reproduce :

1. Add a user 'john'
2. Affect this user 'john' to a board
3. Create a card with a description
4. Delete the user 'john' via rest api
5. Reload the web page
6. Now, you can not open the card which have a description and the user avatar is void.

See javascript_error.log and a screenshot: