bot going offline after some period?

that didn't happen on EC2 hosting ... ah, logs show:

```
]: notice: RTM websocket opened
2017-07-20T20:50:00.626753358Z app[web.1]: /app/server.js:55
2017-07-20T20:50:00.626791424Z app[web.1]: greet = greet.replace('USER_NAME', message.user);
2017-07-20T20:50:00.626818040Z app[web.1]: ^
2017-07-20T20:50:00.626822460Z app[web.1]:
2017-07-20T20:50:00.626824971Z app[web.1]: TypeError: Cannot read property 'replace' of undefined
2017-07-20T20:50:00.626858617Z app[web.1]: at Object. (/app/server.js:55:16)
2017-07-20T20:50:00.626862534Z app[web.1]: at Object.Botkit.botkit.trigger (/app/node_modules/botkit/lib/CoreBot.js:986:49)
2017-07-20T20:50:00.626865447Z app[web.1]: at Object. (/app/node_modules/botkit/lib/SlackBot.js:752:30)
2017-07-20T20:50:00.626868460Z app[web.1]: at Object.Botkit.botkit.trigger (/app/node_modules/botkit/lib/CoreBot.js:986:49)
2017-07-20T20:50:00.626871272Z app[web.1]: at /app/node_modules/botkit/lib/CoreBot.js:1090:32
2017-07-20T20:50:00.626873683Z app[web.1]: at Object.bot.findConversation (/app/node_modules/botkit/lib/Slackbot_worker.js:751:9)
2017-07-20T20:50:00.626893871Z app[web.1]: at /app/node_modules/botkit/lib/CoreBot.js:1086:21
2017-07-20T20:50:00.626897487Z app[web.1]: at next (/app/node_modules/ware/lib/index.js:82:27)
2017-07-20T20:50:00.626900098Z app[web.1]: at Ware.run (/app/node_modules/ware/lib/index.js:88:3)
2017-07-20T20:50:00.626902810Z app[web.1]: at Object.Botkit.botkit.receiveMessage (/app/node_modules/botkit/lib/CoreBot.js:1081:35)
```