NoMethodError: undefined method `key' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1021123/

NoMethodError: undefined method `key' for nil:NilClass
  security/store_report_service.rb:52:in `create_or_find_vulnerability_finding'
    primary_identifier: identifiers_objects[occurrence.primary_identifier.key],
  security/store_report_service.rb:39:in `create_vulnerability_finding'
    vulnerability = create_or_find_vulnerability_finding(occurrence)
  security/store_report_service.rb:34:in `block in create_all_vulnerabilities!'
    create_vulnerability_finding(occurrence)
  security/store_report_service.rb:33:in `each'
    @report.occurrences.each do |occurrence|
  security/store_report_service.rb:33:in `create_all_vulnerabilities!'
    @report.occurrences.each do |occurrence|
...
(65 additional frame(s) were not displayed)

undefined method `key' for nil:NilClass