How can I fix the 'handler is not a function' error with my Firebase callable function?

I'm implementing poke notifications in my app using Firebase Cloud Functions, but I'm running into an error when deploying and calling my function. After deploying with firebase deploy --only functions, I get the following server-side error:

BASHCopied
Unhandled error TypeError: handler is not a function
    at fixedLen (/workspace/node_modules/firebase-functions/lib/v2/providers/https.js:153:37)
    at /workspace/node_modules/firebase-functions/lib/common/providers/https.js:544:32
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)

The relevant parts of my code are as follows:

In my Firebase function (sendPoke):

JAVASCRIPTCopied
const functions = require("firebase-functions");
const admin = require("firebase-admin"); //admin sdk

exports.sendPoke = functions.https.onCall(async (data, context) => {
  const sentBy = context.auth?.uid; // sender
  const sentTo = data.sentTo;       // receiver
  const message = data.message || "Seni dürttü!";
  const currentTime = admin.firestore.Timestamp.now();

  if(!sentBy){
    console.error("Failed to add Sender User");
  }
  if(!sentTo){
    console.error("Submitted User could not be added");
  }
  
  if (!sentBy || !sentTo) {
    throw new functions.https.HttpsError(
      "invalid-argument",
      "Sender and recipient information is required."
    );
  }

  try {
    // Bildirimi gönder
    const payload = {
      notification: {
        title: "A new poke!",
        body: `poked you!`,
      },
      data: {
        sentBy,
        sentTo,
        message,
      },
    };
    //I am suspecting this line
    await admin.messaging().sendToDevice(sentTo, payload); //send notification only one user

    return { success: true, message: "Poke sent." };
  } catch (error) {
    console.error("Poke sent error:", error);
    throw new functions.https.HttpsError("unknown", "Bir hata oluştu.");
  }
});

My index.js file is as follows:

JAVASCRIPTCopied
const functions = require("firebase-functions");
const admin = require("firebase-admin");

// if app already started dont start again
if (!admin.apps.length) {
    admin.initializeApp();
}

// export sendPoke function
const sendPoke = require("./functions/utilities/pokeAndNotifications/sendPoke");

exports.sendPoke = functions.https.onCall(sendPoke);

And here is how I call the function from my Flutter app:

DARTCopied
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter_dotenv/flutter_dotenv.dart';
import 'package:cloud_functions/cloud_functions.dart';

Future<void> sendPokeNotification(String sentTo) async {
  try {
    // call callable function
    final HttpsCallable callable =
        FirebaseFunctions
        .instance
        .httpsCallable('sendPoke');
        
    // send data and receive response
    final response = await callable.call(<String, dynamic>{
      "sentTo": sentTo, // receiver UID
      "message": 'Hi, i poked you!', // message content
    });

    print('Function respond: \\${response.data}');
  } catch (e) {
    print('Send Poke Error: $e');
  }
}

I have also attempted different configurations by wrapping the function either in index.js or in the sendPoke file, as shown below:

Option 1: Wrapping in index.js

JAVASCRIPTCopied
const sendPoke = require("./functions/utilities/pokeAndNotifications/sendPoke");
exports.sendPoke = functions.https.onCall(sendPoke); 

Option 2: Wrapping in sendPoke file

JAVASCRIPTCopied
module.exports.sendPoke = functions.https.onCall(async (data, context) => {
  const sentBy = context.auth?.uid; // sender
  const sentTo = data.sentTo;       // receiver
  const message = data.message || "Seni dürttü!";
  const currentTime = admin.firestore.Timestamp.now();

  if(!sentBy){
    console.error("Failed to add Sender User");
  }
  if(!sentTo){
    console.error("Submitted User could not be added");
  }
  // Remaining code...
});

Combined index.js and sendPoke.js example

JAVASCRIPTCopied
exports.sendPokeYourFriend = functions.https.onCall(async (data, context) => {
  const sentBy = context.auth?.uid; // sender
  const sentTo = data.sentTo;       // receiver
  const message = data.message || "Seni dürttü!";
  const currentTime = admin.firestore.Timestamp.now();

  if(!sentBy){
    console.error("Failed to add Sender User");
  }
  if(!sentTo){
    console.error("Submitted User could not be added");
  }
});

exports.sendPoke = functions.https.onCall(sendPoke.sendPokeYourFriend);

I suspect the error is caused by wrapping the function with functions.https.onCall twice, which results in passing a non-function as the handler. What is the correct way to export and register my callable function so that this error is resolved?