I'm using Compass version 1.15.2. In preparation for demoing pipeline builder at MongoDB.local in Austin, I imported Asya's pipeline.  The pipeline works initially, but if you disable the last stage then re-enable it, you receive an error message saying "Each element of the 'pipeline' array must be an object".

If a re-create the pipeline manually, I do not see this behavior.  

Steps to re-create:

1. In a terminal (with a small font), "cat" the attached file, select the full text for the file and copy to clipboard
2. On Aggregations tab, click ... and select "New Pipeline From Text"
3. Paste the clipboard contents into the text area.
4. Click the "Create New" button
5. On the resulting pipeline, scroll down to the last stage and disable it
6. Re-enable that stage and the error is displayed

I recreated the pipeline manually and it works fine.  

I was using the swagstore database from Asya's Dropbox - I can attack a dump if needed.