class HtmlContent
  include Mongoid::Document

  has_one :user, class_name: 'User', inverse_of: :html_content, validate: false, dependent: :nullify
end

class User
  include Mongoid::Document

  belongs_to :html_content, class_name: 'HtmlContent', inverse_of: :user, dependent: :destroy, validate: false, optional: true
end

user = User.create!
html_content = HtmlContent.create!(user: user)

html_content.destroy! # /lib/mongoid/association/depending.rb:110:in `_dependent_nullify!'
# Raises an error:
# NoMethodError: undefined method `nullify' for #<User _id: 640084c257f2fc7f67cf1862
#
# relation.nullify
# ^^^^^^^^
# /lib/mongoid/association/depending.rb:110:in `_dependent_nullify!'