@david I upgraded to 2.9.0.beta3 which caused the issue.
Regarding the upgrade process, I usually do this via the /admin/upgrade UI. This case was different.
I only upgraded docker-manager from the upgrade UI. After this, I went had to handle some other things, and came visited the /admin/upgrade path a few hours later to complete the upgrade process.
I was met with a text page telling me to go to the host, change to the /var/discourse directory, git pull and rebuild the app with ./launcher…So that is what I did, and then the chaos ensued.
While the stress and nerves were high, I found something on this forum about uncaught TypeError being related to a plugin, so I ripped out everything non-essential from the app.yml plugins and rebuilt the container. At that point, the ember app began working again.