check null safety

lib/data/provider/carton_data_provider.dart

@@ -1,5 +1,4 @@
 import 'dart:async';
-import 'dart:convert';
 
 import 'package:cloud_firestore/cloud_firestore.dart';
 import 'package:fcs/domain/constants.dart';
@@ -36,7 +35,7 @@ class CartonDataProvider {
   }
 
   Future<List<Carton>> searchCarton(String term) async {
-    if (term == null || term == '') return List();
+    if (term == null || term == '') return [];
 
     // var bytes = utf8.encode(term);
     // var base64Str = base64.encode(bytes);
@@ -45,7 +44,7 @@ class CartonDataProvider {
 
     try {
       String path = "/$cartons_collection";
-      var querySnap = await Firestore.instance
+      var querySnap = await FirebaseFirestore.instance
           .collection(path)
           .where("carton_number", isEqualTo: term)
           .where("carton_type",
@@ -53,13 +52,11 @@ class CartonDataProvider {
           .where("status", isEqualTo: carton_packed_status)
           .where("is_deleted", isEqualTo: false)
           .orderBy("user_name")
-          .getDocuments();
-      return querySnap.documents
-          .map((e) => Carton.fromMap(e.data, e.documentID))
-          .toList();
+          .get();
+      return querySnap.docs.map((e) => Carton.fromMap(e.data(), e.id)).toList();
     } catch (e) {
       log.warning("carton error:" + e.toString());
-      return null;
+      return [];
     }
   }
 }