check null safety

lib/helpers/paginator.dart

@@ -13,7 +13,7 @@ class Paginator<T> {
   final log = Logger('Paginator');
 
   final int rowPerLoad;
-  DocumentSnapshot prev;
+  DocumentSnapshot? prev;
   bool ended = false;
   bool isLoading = false;
   List<T> values = [];
@@ -21,7 +21,7 @@ class Paginator<T> {
 
   Query pageQuery;
 
-  Paginator(this.pageQuery, {this.rowPerLoad = 20, this.toObj}) {
+  Paginator(this.pageQuery, {this.rowPerLoad = 20, required this.toObj}) {
     _clearState();
   }
 
@@ -36,27 +36,27 @@ class Paginator<T> {
     _clearState();
   }
 
-  Future<void> refresh({CallBack onFinished}) async {
+  Future<void> refresh({CallBack? onFinished}) async {
     _clearState();
     await load();
     if (onFinished != null) onFinished();
   }
 
-  Future<bool> load({CallBack onFinished}) async {
+  Future<bool?> load({CallBack? onFinished}) async {
     if (ended) return null;
     isLoading = true;
     Query _query =
-        prev != null ? pageQuery.startAfterDocument(prev) : pageQuery;
+        prev != null ? pageQuery.startAfterDocument(prev!) : pageQuery;
     try {
       await _query
           .limit(rowPerLoad)
-          .getDocuments(source: Source.server)
+          .get(GetOptions(source: Source.server))
           .then((QuerySnapshot snapshot) {
-        int count = snapshot.documents.length;
+        int count = snapshot.docs.length;
         ended = count < rowPerLoad;
-        prev = count > 0 ? snapshot.documents[count - 1] : prev;
-        snapshot.documents.forEach((e) {
-          values.add(toObj(e.data, e.documentID));
+        prev = count > 0 ? snapshot.docs[count - 1] : prev;
+        snapshot.docs.forEach((e) {
+          values.add(toObj(e.data() as Map<String, dynamic>, e.id));
         });
       });
     } catch (e) {