This is an easy fix to get a custom launcher back onto your FireStick. Be sure to have Launcher Manager installed and another launcher like Wolf Launcher. And block Amazon update servers so they can't do it again!
d1s31zyz7dcc2d.cloudfront.net
amzdigital-a.akamaihd.net
amzdigitaldownloads.edgesuite.net
softwareupdates.amazon.com
https://forum.xda-developers.com/t/app-firetv-noroot-launcher-manager-change-launcher-without-root.4176349/