EmberJS: Cannot read property '_application' of undefined

I'm working through the emberJS tutorial and every time I update the code it crashes with the following error:

Uncaught TypeError: Cannot read property '_application' of undefined
    at appStarted (<anonymous>:9601:45)
    at Object.initialize (<anonymous>:9721:11)
    at index.js:136
    at Vertices.each (dag-map.js:231)
    at Vertices.walk (dag-map.js:145)
    at DAG.each (dag-map.js:75)
    at DAG.topsort (dag-map.js:83)
    at App._runInitializer (index.js:151)
    at App.runInstanceInitializers (index.js:134)
    at Class._bootSync (instance.js:111)

The only way to get rid of it is to keep restarting the server. Any idea what is going on?