fix: cb is not a function at fallbackErrorHandler

[DzenDyn]

This fixes error if cb is not a function at fallbackErrorHandler:

Tests:

Benchmarks:

Cheklist:

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the [Developer's Certification of Origin](

[mcollina]

Thanks for opening a PR! Can you please add a unit test?

[DzenDyn]

I'm sorry, but I'm afraid I'm not quite sure how to do this.
I have not yet found how to trigger this error and at what point there may not be a callback, but Sentry caught an unhandled exception and the commit I proposed should fix it.

[metcoder95]

I'm starting to guess if your error handler is bad set, I couldn't reproduce it easily.
What I found instead, is that we are not validating properly the error handler when setting fastify.setErrorHandler in contrast to what we do at a route level, where indeed fails if a bad handler is passed.

TypeError: Cannot read properties of undefined (reading 'bind')
    at Object.setErrorHandler (/Users/metcoder/Documents/Workspace/MetCoder/Fastify_Core/fastify.js:847:71)
    at Object.<anonymous> (/Users/metcoder/Documents/Workspace/MetCoder/Fastify_Core/playground.js:5:9)
    at Module._compile (node:internal/modules/cjs/loader:1376:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1435:10)
    at Module.load (node:internal/modules/cjs/loader:1207:32)
    at Module._load (node:internal/modules/cjs/loader:1023:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:135:12)
    at node:internal/main/run_main_module:28:49

[Uzlopak]

Yes, should throw an error when setting the value when starting the webserver and not when the webserver is running.

[Uzlopak]

Closed as it was superseeded by #5317