Projects::MergeRequestsController#discussions: NoMethodError: undefined method `index' for nil:NilClass
https://sentry.gitlab.net/gitlab/gitlabcom/issues/511117/
NoMethodError: undefined method `index' for nil:NilClass
discussion_on_diff.rb:46:in `truncated_diff_lines'
initial_line_index = [diff_line.index - NUMBER_OF_TRUNCATED_DIFF_LINES + 1, 0].max
grape_entity/delegator/plain_object.rb:8:in `delegate'
object.send attribute
grape_entity/entity.rb:512:in `delegate_attribute'
delegator.delegate(attribute)
grape_entity/exposure/delegator_exposure.rb:8:in `value'
entity.delegate_attribute(attribute)
grape_entity/exposure/represent_exposure.rb:27:in `value'
using_class.represent(@subexposure.value(entity, options), new_options)
...
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NoMethodError: undefined method `index' for nil:NilClass