src/hex.h:107:1: error: control reaches end of non-void function

I noticed the switch statement doesn't have a default. It's always a good rule of thumb to include a default statement when using a switch. I added the following and it cleared the error for me. Beware it's untested

default:
return NULL;
}

Open src/hex.h at line 107 and inspect the switch statement that reaches the end of a non-void function. Add the missing fallback return described in the issue, then compile the Arduino library to confirm the error is gone and the switch returns a value on every path.

Written by the indexing model from the issue text.