NoMethodError: undefined method `downcase' for nil:NilClass

https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/992708/

NoMethodError: undefined method `downcase' for nil:NilClass
  banzai/filter/video_link_filter.rb:23:in `has_video_extension?'
    element.attr(src_attr).downcase.end_with?(*UploaderHelper::SAFE_VIDEO_EXT)
  banzai/filter/video_link_filter.rb:12:in `block in call'
    el.replace(video_node(doc, el)) if has_video_extension?(el)
  nokogiri/xml/node_set.rb:238:in `block in each'
    yield self[x]
  nokogiri/xml/node_set.rb:237:in `upto'
    0.upto(length - 1) do |x|
  nokogiri/xml/node_set.rb:237:in `each'
    0.upto(length - 1) do |x|
...
(166 additional frame(s) were not displayed)

NoMethodError: undefined method `downcase' for nil:NilClass