API resolved without sending a response in Nextjs

You should return a Promise and resolve/reject it.

Example:

import { getData } from "../../helper";

export default async function(req, res) {
  return new Promise((resolve, reject) => {
    getData()
      .then(response => {
        res.statusCode = 200
        res.setHeader('Content-Type', 'application/json');
        res.setHeader('Cache-Control', 'max-age=180000');
        res.end(JSON.stringify(response));
        resolve();
      })
      .catch(error => {
        res.json(error);
        res.status(405).end();
        resolve(); // in case something goes wrong in the catch block (as vijay commented)
      });
  });
};