Flutter/Dart FutureBuilder: NoSuchMethodError: The method '[]' was called on null

Question

I have the following problem. Whenever the futurebuilder is re-executed, the following error appears in the terminal

NoSuchMethodError: The method '[]' was called on null

Nevertheless, the app works because an exception is always made.

Unfortunately, I have not found a suitable answer yet.

void initState(){                                                         
    _getdata = getdata();                                               
    super.initState();
}

Future getdata() async {                                                
   DocumentReference qn = Firestore.instance.collection("data").document("datatxt");                               
   return qn.get();                                                         
}

...
child: FutureBuilder(
            future: _getdata,
             builder: (BuildContext context, snap){
                  return Container(
                        width: MediaQuery.of(context).size.width,
                        color: Colors.green,
                        child: Text(snap.data['Text1']),
                      );                 
                    },
                  ),

Another exception was thrown: NoSuchMethodError: The method '[]' was called on null

Answer 1

When you work with dart Futures (async/await) on Flutter you should prepare your widgets for 3 possible states.

1. Uncompleted
2. Completed with a value
3. Completed with an error

I suggest you to work with FutureBuilder, StreamBuilder works the same way too, by checking the ConnectionState of the snapshot inside de builder function.

I use this approach:

 FutureBuilder(
     future: _getdata,
     builder: (BuildContext context, AsyncSnapshot snap){
         switch (snapshot.connectionState) {
             // Uncompleted State
             case ConnectionState.none:
             case ConnectionState.waiting:
                 return Center(child: CircularProgressIndicator());
             break;
             default:
                 // Completed with error
                 if (snapshot.hasError)
                     return Container(Text(snapshot.error.toString()));

                 // Completed with data
                 return Container(
                        width: MediaQuery.of(context).size.width,
                        color: Colors.green,
                        child: Text(snap.data['Text1']),
                        );
)

References Youtube

FutureBuilder

Dart Futures