Hi there,

I have an error "Cannot read property 'high_' of null" when using aggregate and setting event listeners on the cursor. Here is the code to reproduce the issue:
const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'test';
const client = new MongoClient(url);

// Use connect method to connect to the server
client.connect((err) => {
assert.equal(null, err);
console.log('Connected successfully to server');

const db = client.db(dbName);

const col = db.collection('test');

col.updateOne(
{
_id: '1',
},
{ $set: { duration: 10 } },
{ upsert: true },
)
.then(() => {
col.aggregate([
{
$group: {
_id: null,
totalDuration: { $sum: '$duration' },
},
},
], (err, cursor) => {

// remove this listener and the exception does not throw
cursor.on('data', () => console.log('cursor data'));

cursor.toArray((err, documents) => {
console.log(documents);
});
});
});
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