NoMethodError: undefined method `start_with?' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/4025068/?referrer=gitlab_plugin

NoMethodError: undefined method `start_with?' for nil:NilClass
  lib/gitlab/github_import/markdown/attachment.rb:58:in `github_url?'
    url.start_with?(::Gitlab::GithubImport::MarkdownText::GITHUB_MEDIA_CDN)
  lib/gitlab/github_import/markdown/attachment.rb:50:in `from_inline_html'
    return unless github_url?(img[:src], media: true)
  lib/gitlab/github_import/markdown/attachment.rb:18:in `from_markdown'
    from_inline_html(markdown_node)
  lib/gitlab/github_import/markdown_text.rb:56:in `extract_attachment'
    ::Gitlab::GithubImport::Markdown::Attachment.from_markdown(node)
  lib/gitlab/github_import/markdown_text.rb:47:in `block in fetch_attachments'
    attachment = extract_attachment(node)
...
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undefined method `start_with?' for nil:NilClass