Regarding your question about callouts, note that there is a regression since runtime version 4.83+ that is preventing callouts from showing. We’ll announce once this is fixed.
You could test callouts in an earlier runtime version. Your code is nearly there. You are receiving the null error because view.selected_user is null. It’s null because the view.users query is missing an await.
You should be able to update the corresponding code to:
view.users = await DB.user.all().toArray(); // also update view.users type to array
view.selected_user = view.users[0];
And in your showCallouts function add a // @ts-ignore to the column property, and assign it a number, e.g.: