I've noticed that preview in both HTML5 and Node Webkit has been failing in R188:
Assertion failure: Project model unavailable
Stack trace:
assert2@http://[MYIP]/preview_prelude.js:16:10
Runtime.prototype.loadProject@http://[MYIP]/preview.js:1089:3
Runtime.prototype.requestProjectData/xhr.onload@http://[MYIP]/preview.js:375:5
Subsequent failures will now be logged to the console.
This project worked fine in r184 so it's pretty strange.