Unhandled rejection MongoError: server instance pool was destroyed

[abhhiram]

I have generated 5 migration files using stub generator. while executing the migration directory using migrator.runFromDir while running files it throw this error "Unhandled rejection MongoError: server instance pool was destroyed" for all the files.

there is my code...

var mm = require('mongodb-migrations');
var path = require('path');
var dir = path.join(__dirname, '1.3.5');
var config = require('./mm-config.json'),
mongo=require('mongoose'),

var migrator = new mm.Migrator(config, customLogFn);

migrator.runFromDir(dir, doneFn, progressFn);
migrator.migrate(doneFn, progressFn);
migrator.dispose(cb);

function cb(error) {
customLogFn("system", "AFTER_DISPOSE ERROR: " + error);
}
function customLogFn(level, message) {
console.log(message);
}
function doneFn(error, results) {
console.log(results}
function progressFn(id, result) {
console.log("MIGRATOR_FILE:", id, result);
}

Please help me out
Thank you.