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

1. Go to the Pipelines tab on a Merge Request where the branch is from a downstream fork (like terraform-images!132 (merged))
2. 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)

Possible fixes