Slack misses tray indicator

I have a fresh installation of Ubuntu 18.04, I installed Slack but it doesn't show any icon in the tray indicator:

▶ snap info slack       
name:      slack
summary:   Team communication for the 21st century.
publisher: slack
contact:   https://get.slack.help/hc/en-us
license:   unknown
description: |
  Caution: Slack for Linux is in beta. We’re still busy adding features and
  ironing out potential issues.

  Slack brings team communication and collaboration into one place so you can
  get more work done, whether you belong to a large enterprise or a small
  business. Check off your to-do list and move your projects forward by bringing
  the right people, conversations, tools, and information you need together.
  Slack is available on any device, so you can find and access your team and
  your work, whether you’re at your desk or on the go.

  Scientifically proven (or at least rumored) to make your working life simpler,
  more pleasant, and more productive. We hope you’ll give Slack a try.

  Stop by and learn more at: https://slack.com/
commands:
  - slack
snap-id:   JUJH91Ved74jd4ZgJCpzMBtYbPOzTlsD
tracking:  stable
refreshed: 2018-03-31T00:45:51+02:00
installed:   3.1.1 (6) 98MB classic
channels:              
  stable:    3.1.1 (6) 98MB classic
  candidate: ↑              
  beta:      ↑              
  edge:      3.1.1 (6) 98MB classic