What is the exact error? As far as I see in the code, these handlers are checked for null before calling them. Maybe problem is with example?

The application crashes, segfaults. I ran it on Android 15 under Termux (clang) and Linux (probably clang again) where it crashed.

Using gdb, the crash is caused by assigning null to a std::function.

The crash can be reproduced on Lunux by compiling with clang and just using example-chat, the faulty code is in Crow, not the example.

The check for null isn't the problem, it's assigning null that's the problem.