check whether container exists before appending icon

[brumm]

This fixes an uncaught exception if the status-bar does not exist due to the entire package being disabled

fixes #178

To make the maintainer's life easier, I...

• [x] created at least 1 spec to cover my changes
• [x] npm tested my code and it's green like an :green_apple:

[balupton]

this will be great, the current theme I use hides the status bar, so I disabled the package, but getting an error, so this pull request will help!

[florianb]

Thank you very much for your contribution @brumm! 🎁