Unhandled Exception: Null check operator used on a null value

[user97116]

/flutter ( 4415): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Null check operator used on a null value
E/flutter ( 4415): #0 BoardViewState.build. (package:boardview/boardview.dart:570:53)
E/flutter ( 4415): #1 _RootZone.run (dart:async/zone.dart:1655:54)
E/flutter ( 4415): #2 _FutureListener.handleWhenComplete (dart:async/future_impl.dart:199:18)
E/flutter ( 4415): #3 Future._propagateToListeners.handleWhenCompleteCallback (dart:async/future_impl.dart:810:39)
E/flutter ( 4415): #4 Future._propagateToListeners (dart:async/future_impl.dart:866:11)
E/flutter ( 4415): #5 Future._completeWithValue (dart:async/future_impl.dart:641:5)
E/flutter ( 4415):
E/flutter ( 4415):