NoMethodError: undefined method `index' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/739466/

NoMethodError: undefined method `index' for nil:NilClass
  gitlab/diff/file.rb:103:in `diff_hunk'
    diff_line_index = diff_line.index
  diff_note.rb:39:in `create_diff_file'
    .merge(diff: diff_file.diff_hunk(diff_line))
  gitlab/metrics/instrumentation.rb:161:in `block in create_diff_file'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `create_diff_file'
    .measure { super }
...
(48 additional frame(s) were not displayed)

undefined method `index' for nil:NilClass