Error when loading new level

[richrd]

This is the first time this has happened. I tried to enter level 4 and the app crashed at the loader screen with the following error:

Uncaught Error: Invalid argument: null
    at Object.wrapException (:8080/main.dart.js:3550)
    at Interceptor.$gt (:8080/main.dart.js:1202)
    at Object.J.$gt$n (:8080/main.dart.js:30145)
    at $async$generate$1 (:8080/main.dart.js:14140)
    at :8080/main.dart.js:6009
    at _SyncStarIterator.moveNext$0 (:8080/main.dart.js:6014)
    at LoadingDialog.update$0 (:8080/main.dart.js:25881)
    at UserInterface.refresh$0 (:8080/main.dart.js:27684)
    at UserInterface.dart.UserInterface._tick$1 (:8080/main.dart.js:27713)
    at Object.eval (eval at Closure_forwardCallTo (:8080/main.dart.js:3866), <anonymous>:2:39)