I keep getting Unexpected end of JSON input error when I test and it keeps crashing

import express from 'express';
import mongoose from 'mongoose';
import Messages from './dbMessages.js'

const app = express ()
const port = process.env.PORT || 9000;

app.use(express.json());

const connection_url = '...'

mongoose.connect(connection_url, {
    useNewUrlParser: true,
    useUnifiedTopology: true
})

app.get('/', (req, res)=> res.status(200).send('hello world'))

app.get('/messages/sync', (req, res) => {
    Messages.find((err, data) => {
        if(err) {
            res.status(500).send(err)
        } else {
            res.status(200).send(data)
        }
    })
})

app.post('/messages/new', (req, res) => {
    const dbMessage = req.body

    Messages.create(dbMessage, (err, data) => {
        if (err) {
            res.status(500).send(err)
        } else {
            res.status(201).send(data)
        }
    })
})

app.listen(port, ()=>console.log(`Listening on localhost:${port}`))

with all these I don't know why I keep getting an error each time I test the get method using postman,