Flutter - The getter 'isNotEmpty' was called on null. what exactly is causing this error

Question

class Logs extends StatefulWidget {
  const Logs({
    Key key,
  }) : super(key: key);

  @override
  State<Logs> createState() => _LogsState();
}

class _LogsState extends State<Logs> {
  String userId;
  userId = firebaseUser.uid;
 

 
  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
        stream: FirebaseFirestore.instance.collection(userId).snapshots(),
        builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if (snapshot.hasError) {
            print('Something went Wrong');
          }
          if (snapshot.connectionState == ConnectionState.waiting) {
            return Center(
              child: CircularProgressIndicator(),
            );
          }

          final List storedocs = [];
          snapshot.data.docs.map((DocumentSnapshot document) {
            Map a = document.data() as Map<String, dynamic>;
            storedocs.add(a);
            a['id'] = document.id;
          }).toList();
          var n = storedocs.length;
          return Scaffold(
            appBar: AppBar(
                title: Text("Smart Electric"),
                foregroundColor: Colors.white,
                backgroundColor: Colors.yellow[600]),
            body: Container(
                  children: <Widget>[
                    if (storedocs.isEmpty) ...{
                      Text(" No readings Received yet"),
                    } else ...{
                      for (int i = 0; i < n; i++) ...[
                        // if (storedocs[i]['UserId'] == userId) ...{
                        logss(storedocs[i]['Units'],
                            storedocs[i]['Time'].toDate(), storedocs[i]['id']),
                        //  }
                      ],
                    }
                  ],
                ),
              ),
            ),
          );
        });
  }
}

════════ Exception caught by widgets library ═══════════════════════════════════

The getter 'isNotEmpty' was called on null. Receiver: null Tried calling: isNotEmpty

Following is the exception which is being caught,what thing is causing this exception?