Regression: Error Tracking page fails with 500

Summary

The Error Tracking page ( -> Operation -> Error Tracking) is failing with a 500 error, as a result of !19137 (merged).

The error is as follows:

NoMethodError at /root/monitor-sandbox/error_tracking.json
==========================================================

> private method `external_url' called for #<ErrorTracking::ListIssuesService:0x00007f93b7e006d0>

app/controllers/projects/error_tracking_controller.rb, line 53
--------------------------------------------------------------

``` ruby
   48   
   49       return if handle_errors(result)
   50   
   51       render json: {
   52         errors: serialize_errors(result[:issues]),
>  53         external_url: service.external_url
   54       }
   55     end
   56   
   57     def render_issue_detail_json
   58       service = ErrorTracking::IssueDetailsService.new(project, current_user, issue_details_params)

Steps to reproduce

Setup Sentry error tracking. Go to ( -> Operation -> Error Tracking).

What is the current bug behavior?

Page fails to load data.

What is the expected correct behavior?

Page should load data.