Streambuilder - error after flutter migration - snapshot syntax

Question

I used this code bevor migration

StreamBuilder(
          stream: FirebaseDatabase.instance
              .ref()
              .child('user')
              .child(_userID)
              .onValue,
          builder: (context, snapshot) {
            if (snapshot.hasData) {
              _networkImageUrl = snapshot.data.snapshot.value["img"];
              return Text(_networkImageUrl.toString());
             } else {
               return Container();
            }
          }),

after "dart pub upgrade --null-safety" I get the error:

The property 'snapshot' can't be unconditionally accessed because the receiver can be 'null'.

I tried to fix it with "!" but it doesn't work, it keeps the same error