Morecast: Crash when forecasting 'quickly'

[brettedw]

Actual Behaviour

Entering values or double clicking to forecast in quick succession can result in a crash of morecast

Steps To Reproduce On dev machines

1. Throttle CPU w/ browser dev tools
2. Enter values in quick succession in successive rows
3. App should crash with Uncaught TypeError: Cannot assign to read only property 'choice' of object '#<Object>'

Expected Behaviour

A clear and concise description of what you expected to happen.