Flutter - The method 'then' was called on null

Question

I have a flutter project with 2 functions:

Future<dynamic> eval(String js) =>
  controller?.evaluateJavascript(js)?.catchError((_) => '');

void _getBottomPosition() async {
    final evals = await Future.wait([
        eval("(window.innerHeight + window.scrollY) >= document.body.offsetHeight"),
    ]);
    String bottmPosition = evals[0].toString() ?? '';
    isBottomPosition = bottmPosition;
    print("bottomPosition: " + isBottomPosition);
}

I'm getting the following error:

E/flutter (14430): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'then' was called on null.
E/flutter (14430): Receiver: null
E/flutter (14430): Tried calling: then<Null>(Closure: (dynamic) => Null, onError: Closure: (dynamic, StackTrace) => Null)

Can anyone help me solve this?

Thanks.

I don't really know Dart, but could take a guess from a different async background: Your method *eval* is not async, so it will return *null* as a result if controller is null, not a Future of null. This might be a problem for *Future.wait*, which will effectively become `Future.wait([null])` — Dennis, Jan 15 '20 at 07:33
... and if that guess is correct, you could resolve it by adding a default future to your eval implementation, like `controller?.evaluateJavascript(js)?.catchError((_) => '') ?? Future.value(null)`, as described here: https://stackoverflow.com/questions/18423691/dart-how-to-create-a-future-to-return-in-your-own-functions But again, I have little experience with Dart. — Dennis, Jan 15 '20 at 07:41
Future eval(String js) async{ return await controller?.evaluateJavascript(js)?.catchError((_) => '');} — Ketan Ramteke, Jan 15 '20 at 07:42

score 1 · Answer 1 · answered Jan 15 '20 at 07:49

You should try this :

Future<dynamic> eval(String js) async =>
  controller?.evaluateJavascript(js)?.catchError((_) => '');

void _getBottomPosition() async {
    final eval = await eval("(window.innerHeight + window.scrollY) >= document.body.offsetHeight");
    String bottmPosition = eval.toString() ?? '';
    isBottomPosition = bottmPosition;
    print("bottomPosition: " + isBottomPosition);
}

First add async keyword to eval method then update eval call in _getBottomPosition.

Flutter - The method 'then' was called on null

1 Answers1