NoMethodError: undefined method `namespace' for nil:NilClass

https://sentry.gitlap.com/gitlab/gitlabcom/issues/250962/

NoMethodError: undefined method `namespace' for nil:NilClass
  submodule_helper.rb:88:in `relative_self_links'
    namespace = @project.namespace.full_path
  submodule_helper.rb:34:in `submodule_links'
    relative_self_links(url, submodule_item.id)
  diff_file_entity.rb:146:in `block in memoized_submodule_links'
    submodule_links(diff_file.blob, diff_file.content_sha, diff_file.repository)
  gitlab/utils/strong_memoize.rb:26:in `strong_memoize'
    instance_variable_set(ivar(name), yield)
  diff_file_entity.rb:144:in `memoized_submodule_links'
    strong_memoize(:submodule_links) do
...
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NoMethodError: undefined method `namespace' for nil:NilClass