Change collection with node-mongodb-native

Question

I want change collections with another query's result.

MongoClient.connect(mongo_url, function (err, db) {
        db.collection('col1').find({}).limit(1).toArray(function (err, docs) {
            if(_.isEmpty(docs)) {
                db.collection('col2').find({})....
            } else {
                db.collection(docs[0].colname).find({})....
            }    
        });
    });

like this.

but that occurred this error

{ [MongoError: Topology was destroyed] name: 'MongoError', message: 'Topology was destroyed' }

How can i solve this problem?