Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client | body.content undefined

Question

Why am I getting this error message?

This is the only route via post. As far as I can see there is no other place where a header is sent.

I also don't understand why body.content evaluates to undefined,even though an object is sent and the header is set to json. By the way, the body parser or app.use(bodyParser.json()) didn't do anything.

"express": "^4.17.3"

I am thankful for help

const express = require("express");
const app = express();
app.use(express.json());

app.post("/api/persons/", (req, res) => {
  const body = req.body;
  //if body has no data return here
  console.log(body.content);
  if (!body.content) {
    return res.status(400).end().json({ error: "missing content" });
  }
//generate entry in list
  const person = {
    number: body.number,
    name: body.name,
    id: generateID(),
  };
  persons = persons.concat(person);
  res.json(person);
});