onRun handler breaks in Sketch 49
![image](https://user-images.githubusercontent.com/1253402/36991921-19f4c366-205e-11e8-8983-ce733186177d.png)

I first noticed it broke on a plugin I created two weeks before the release of Sketch 49.  I've reproed this by creating a new plugin with the latest version of `skpm`.  

Not sure how to address this as I'm not too familiar with the builder code.