NoMethodError: undefined method `sha' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/4091743/?referrer=gitlab_plugin

NoMethodError: undefined method `sha' for nil:NilClass
  app/models/repository.rb:963:in `root_ref_sha'
    @root_ref_sha ||= commit(root_ref).sha
  app/services/security/ci_configuration/base_create_service.rb:86:in `root_ref_sha'
    project.repository.root_ref_sha
  app/services/security/ci_configuration/base_create_service.rb:54:in `existing_gitlab_ci_content'
    root_ref = root_ref_sha(project)
  app/services/security/ci_configuration/sast_create_service.rb:23:in `action'
    Security::CiConfiguration::SastBuildAction.new(project.auto_devops_enabled?, params, existing_gitlab_ci_content, project.ci_config_path).generate
  app/services/security/ci_configuration/base_create_service.rb:49:in `attributes'
    actions: [action]
...
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