Delete User does not work

[SaturnHafen]

Bug: Delete user button does not work

Problem Description

When trying to delete the user account, an error occurs.

Steps For Reproduction

1. Create user account
2. Sign in
3. Delete the user account

Make sure to check if the error is different when the user has whishlist-items or owns items

[SaturnHafen]

While working on #205 we discovered this problem (and we will add a new relation which might contain the same problem). When fixing this issue please talk to @antonykamp and @SaturnHafen