Starting discussion from user profile fails when using custom user slug driver

[clarkwinkelmann]

Bug Report

Current Behavior
If the slug driver is set to something different than username, Flarum will crash with a javascript error when clicking the "Start private discussion" link on a user profile

Steps to Reproduce

1. Set user slug driver to ID
2. Visit any user profile
3. Use dropdown button to start private discussion with user

Error loosely transcripted from screenshot:

TypeError: Cannot read properties of undefined (reading 'id') at addDefaultRecipients (PrivateDiscussionComposer.js)

Expected Behavior
Should work with any slug driver.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment

• Flarum version: 1.4.0
• Extension version: master

Output of "php flarum info", run this in terminal in your Flarum directory.

Possible solution(s)

byobu/js/src/forum/pages/discussions/PrivateDiscussionComposer.js

Line 107 in bc5dafa

const user = app.store.getBy('users', 'username', username);

I think just replacing 'username' with 'slug' as the second parameter should fix it.

This would probably be a good opportunity to gracefully handle null being returned by getBy even though it supposedly shouldn't happen.

Additional Context
Based on report here https://discuss.flarum.org/d/4762-friendsofflarum-by-bu-well-integrated-advanced-private-discussions/1166