Fix 500 when reviewer is deleted with integration tests

[lunny]

will fix #6797

[codecov-io]

Codecov Report

Merging #6856 into master will increase coverage by 0.03%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #6856      +/-   ##
==========================================
+ Coverage   41.14%   41.17%   +0.03%     
==========================================
  Files         425      425              
  Lines       58484    58489       +5     
==========================================
+ Hits        24061    24082      +21     
+ Misses      31236    31220      -16     
  Partials     3187     3187

Impacted Files	Coverage Δ
models/issue_comment.go	47.48% <100%> (+0.57%)	⬆️
routers/repo/issue.go	35.87% <42.85%> (+0.29%)	⬆️
models/unit.go	62.16% <0%> (-5.41%)	⬇️
routers/repo/view.go	42.02% <0%> (-1.02%)	⬇️
routers/user/home.go	49.89% <0%> (+0.63%)	⬆️
routers/repo/pull.go	36.86% <0%> (+0.72%)	⬆️
models/issue_comment_list.go	75.62% <0%> (+1.74%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dab38c3...ed253d8. Read the comment docs.