Run pipeline button on Pipelines tab of Merge Request is broken

Summary

When on the Pipelines tab of a Merge Request where the branch is from a downstream fork, clicking the "Run pipeline" button does not work, and results in an error in the browser developer tools console.

Steps to reproduce

Go to the Pipelines tab on a Merge Request where the branch is from a downstream fork (like terraform-images!132 (merged))
Click the "Run pipelines" button

Example Project

terraform-images!132 (merged)

What is the current bug behavior?

Nothing visibly happens, this error is produced in the developer tools console:

TypeError: this.$refs.modal is undefined
    tryRunPipeline pipelines_table.vue:176
    VueJS 5
    r helpers.js:72
    _wrapEventTarget trycatch.js:137
    jt instrument.js:365
    VueJS 51
    storePipelines pipelines_store.js:16
    setCommonData pipelines_mixin.js:144
    successCallback pipelines_table.vue:146
    checkConditions poll.js:87
    makeRequest poll.js:106
    promise callback*makeRequest poll.js:105
    beforeMount pipelines_mixin.js:39
    VueJS 15
instrument.js:109:45

What is the expected correct behavior?

A modal confirmation explaining risks of running a pipeline from a downstream fork is displayed with a button that allows you to confirm and trigger a pipeline.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info


(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)