404 when assigning a ticket to a user which had been assigned to a group

[hi-ko]

seen with Plugin version 3.1.2 / Redmine 4.0.4.stable.18593

How to reproduce:

• Assign a ticket to a group user (needs to be activated in Redmine settings:
  
  In our case this groupuser has ID=4
• change Assignee to a "normal" user
  system throws an ERROR and returns the user a http 404:

Processing by IssuesController#update as HTML
  Parameters: {"utf8"=>"â<9c><93>", "authenticity_token"=>"**************************", "form_update_triggered_by"=>"", "issue"=>{"is_private"=>"0", "project_id"=>"164", "author_id"=>"392", "tracker_id"=>"3", "subject"=>"Missing german locale for share actions", "description"=>"The actions labels in Share are currently only available in english. We should add a german translation.", "status_id"=>"1", "priority_id"=>"4", "assigned_to_id"=>"392", "fixed_version_id"=>"", "parent_issue_id"=>"", "start_date"=>"2020-05-15", "due_date"=>"", "estimated_hours"=>"", "done_ratio"=>"0", "custom_field_values"=>{"12"=>""}, "tag_list"=>[""], "notes"=>"Hi Sinan,\r\ncould you please take care of such little things right away?", "private_notes"=>"0", "lock_version"=>"0"}, "was_default_status"=>"1", "time_entry"=>{"hours"=>"", "activity_id"=>"", "comments"=>""}, "last_journal_id"=>"", "commit"=>"Submit", "id"=>"7976"}
  Current user: ********.******* (id=3)
Completed 404 Not Found in 77ms (ActiveRecord: 19.6ms)

ActiveRecord::RecordNotFound (Couldn't find User with 'id'=4 [WHERE `users`.`type` IN ('User', 'AnonymousUser')]):

plugins/redmine_zulip/lib/redmine_zulip/issue_patch.rb:296:in `update_issue_subject'
app/controllers/issues_controller.rb:552:in `save_issue_with_child_records'
app/controllers/issues_controller.rb:169:in `update'
lib/redmine/sudo_mode.rb:63:in `sudo_mode'