MacBook doesn't find any Bluetooth device
I had the same problem, after inserting an external bluetooth dongle my Macbook pro 2018 internal bluetooth control has gone, tried everything, resetting SMC, NVRAM, installing a bran new Mojave on an USB disk
I opened a ticket to Apple and after a morning of retries they concluded my internal controller hardware is broken.

I suggest you to run "Bluetooth Explorer" (I was able but "Bluetooth Explorer" does not detect my internal controller..)