I have a class that have one method .I want change return type of my method to promise . but in promise can not access to properties of class .how can resolve this .But get this exception 

**Reason: TypeError: Cannot read property 'bot' of undefined**

```
const SDK = require('balebot');
const Promise = require('bluebird');

import incoming from './incoming';
const _ = require('lodash');

class Bale {
  constructor(bp, config) {
    if (!bp || !config) {
      throw new Error('You need to specify botpress and config');
    }
    this.bot = null;
    this.connected = false;
    this.bot = new SDK.BaleBot(config.botToken);
    bp.logger.info('bale bot created');
  }

  setConfig(config) {
    this.config = Object.assign({}, this.config, config);
  }

  sendText(chat, text, options) {
    let msg = new SDK.TextMessage(text);

    return new Promise(function (resolve, reject) {
      var response = this.bot.send(msg, receiver);
      if (response) {
        reject(err);
      } else {
        resolve(response);
      }
    });
  }

}

module.exports = Bale;

```