Flutter/Dart - Suggestions for how to Troubleshoot a Future?

Though it was working previously, for some reason my code has now stopped working. Though it fetches the required json data, the build doesn’t render. The error on my app page which is supposed to display a page view was;

type String is not a subtype of 'Map<String,dynamic>' 

But after some tweaks, now the error is;

invalid arguments(s)

I think I may have narrowed it down to the Future;

FutureBuilder<List<SpeakContent>> futureStage() {
  return new FutureBuilder<List<SpeakContent>>(
    future: downloadJSON(),
    builder: (context, snapshot) {
      if (snapshot.hasData) {
       print("Snapshot has data.");
        List<SpeakContent> speakcrafts = snapshot.data;
        return new CustomPageView(speakcrafts);
      } else if (snapshot.hasError) {
        return Text('${snapshot.error}');
      }
      return new CircularProgressIndicator();
    },
  );
}

Future<List<SpeakContent>> downloadJSON() async {
  final jsonEndpoint =
      "http://example.com/getContent.php?";    
  final response = await get(jsonEndpoint);

  if (response.statusCode == 200) {    
    List speakcrafts = json.decode(response.body);
    debugPrint(speakcrafts.toString());
    return speakcrafts
        .map((speakcraft) => new SpeakContent.fromJson(speakcraft))
        .toList();
  } else
    throw Exception('We were not able to successfully download the json data.');
}

Although it doesn’t throw an error, I’ve noticed that it doesn’t print my test statement after the "if (snapshot.hasData)" line.

Shouldn’t I see "Snapshot has data." appear in my Android Studio console?

type String is not a subtype of 'Map<String,dynamic>' 

return Text('${snapshot.error}');