desktop: Tray icon on Ubuntu is missing

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• This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/.
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Summary Tray icon on Ubuntu 17.10 is not shown

Steps to reproduce I’ve installed the mattermost client with the .deb package. Configured my server and then try to configure the client itself. But at both options, Light and Dark, the tray icon is not shown.

• Operating System: Ubuntu 17.10 - 64 Bit - Kernel 4.13.0-38-generic
• Mattermost Desktop App version: 4.0.1
• Mattermost Server version: Mattermost Team Edition 4.7.1 (4.7.3) with postgres

Expected behavior Show the tray icon.

Observed behavior The fields trayIconTheme and showTrayIcon in the ~/.config/Mattermost/config.json are set correctly.

Config:

{
  "teams": [
    {
      "name": "xxx",
      "url": "https://xxx"
    }
  ],
  "showTrayIcon": true,
  "trayIconTheme": "dark",
  "version": 1,
  "minimizeToTray": true,
  "notifications": {
    "flashWindow": 2,
    "bounceIcon": false,
    "bounceIconType": "informational"
  },
  "showUnreadBadge": true,
  "useSpellChecker": true,
  "spellCheckerLocale": "de-DE"
}

Possible fixes -/-