Old group label don't disappear - Sketch 51.2
We use the plugin to keep our design library tidy and easy to scan. 
Since a couple of weeks we noticed that some old group label are stuck on the symbol page. They wont't disappear and I can't deleted them. They are not in any level. 
<img width="437" alt="screenshot at sep 04 17-13-39" src="https://user-images.githubusercontent.com/38939310/45043882-5db38900-b066-11e8-91b5-caa7a47874de.png">

