remove assignee on issue list return 500

[Cherrg]

• Gitea version (or commit ref): master branch
• Git version: latest build (master)
• Operating system: Linux/Ubuntu 18.04
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
  • irrelevant
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (https://try.gitea.io/mgnehr/test/issues)
  • No
  • Not relevant
• Log gist:
  2019/06/29 19:47:07 ...outers/repo/issue.go:1127:UpdateIssueAssignee() [E] ChangeAssignee: UpdateIssueUserByAssignee: user does not exist [uid: 0, name: , keyid: 0]

Description

remove assignee on issue list return 500

Screenshots