check null safety

lib/pages/chat/bubble.dart

@@ -2,7 +2,6 @@ import 'package:fcs/helpers/theme.dart';
 import 'package:fcs/pages/package/package_info.dart';
 import 'package:fcs/pages/main/util.dart';
 import 'package:fcs/pages/widgets/fcs_id_icon.dart';
-import 'package:fcs/pages/widgets/local_text.dart';
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
@@ -14,18 +13,18 @@ typedef CallbackOnViewDetail();
 class Bubble extends StatelessWidget {
   Bubble(
       {this.message,
-      this.date,
-      this.delivered,
-      this.isMine,
+      required this.date,
+      required this.delivered,
+      required this.isMine,
       this.sender,
-      this.isSystem,
-      this.isCustomer,
-      this.showDate,
+      required this.isSystem,
+      required this.isCustomer,
+      required this.showDate,
       this.callbackOnViewDetail});
 
-  final CallbackOnViewDetail callbackOnViewDetail;
+  final CallbackOnViewDetail? callbackOnViewDetail;
   final DateTime date;
-  final String message, sender;
+  final String? message, sender;
   final bool delivered, isMine, isSystem, isCustomer, showDate;
 
   @override
@@ -84,7 +83,7 @@ class Bubble extends StatelessWidget {
                                     fontWeight: FontWeight.bold)))
                       ]
                     : [
-                        Text(isCustomer ? "FCS Team" : sender,
+                        Text(isCustomer ? "FCS Team" : sender ?? "",
                             style: TextStyle(
                               color: Colors.black38,
                               fontSize: 10.0,
@@ -102,8 +101,8 @@ class Bubble extends StatelessWidget {
       children: <Widget>[
         Padding(
             padding: EdgeInsets.only(right: 48.0),
-            child: Text(message,
-                style: hasUnicode(message)
+            child: Text(message ?? "",
+                style: hasUnicode(message ?? "")
                     ? newLabelStyleMM(color: primaryColor)
                     : newLabelStyle(color: primaryColor))),
         Positioned(
@@ -130,6 +129,6 @@ class Bubble extends StatelessWidget {
   }
 
   _viewDetail() {
-    if (callbackOnViewDetail != null) callbackOnViewDetail();
+    if (callbackOnViewDetail != null) callbackOnViewDetail!();
   }
 }