phase3: util_web + shop_scroll (custom scroll) null-safe; global Key? param fix

lib/widgets/general/carousel.dart

@@ -102,7 +102,7 @@ class CarouselState extends State<Carousel> {
 }
 
 class Indicator extends StatefulWidget {
-  Indicator({Key key, int count})
+  Indicator({Key? key, int count})
       : count = count,
         super(key: key);
 

lib/widgets/general/download_item.dart

@@ -13,7 +13,7 @@ import '../../utils/utils.dart';
 class DownloadItem extends StatefulWidget {
   final dynamic desc;
   final double width;
-  const DownloadItem(this.desc, {Key key, this.width}) : super(key: key);
+  const DownloadItem(this.desc, {Key? key, this.width}) : super(key: key);
 
   @override
   State<StatefulWidget> createState() {

lib/widgets/general/hover_widget.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 
 /// 自定义实现，替代已停维的 `hovering` 包的 [HoverWidget]。

lib/widgets/general/index_carousel.dart

@@ -7,7 +7,7 @@ import 'package:responsive_builder/responsive_builder.dart';
 
 class IndexCarousel extends StatelessWidget {
   final List<Gallery> galleries;
-  const IndexCarousel(this.galleries, {Key key}) : super(key: key);
+  const IndexCarousel(this.galleries, {Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {

lib/widgets/general/index_main_content_1.dart

@@ -6,7 +6,7 @@ import 'package:responsive_builder/responsive_builder.dart';
 
 class IndexMainContent1 extends StatelessWidget {
   final String message;
-  const IndexMainContent1(this.message, {Key key}) : super(key: key);
+  const IndexMainContent1(this.message, {Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {

lib/widgets/general/index_main_content_2.dart

@@ -6,7 +6,7 @@ import 'package:responsive_builder/responsive_builder.dart';
 
 class IndexMainContent2 extends StatelessWidget {
   final Map<String, dynamic> content;
-  const IndexMainContent2(this.content, {Key key}) : super(key: key);
+  const IndexMainContent2(this.content, {Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {

lib/widgets/general/index_main_content_3.dart

@@ -6,7 +6,7 @@ import 'package:responsive_builder/responsive_builder.dart';
 
 class IndexMainContent3 extends StatelessWidget {
   final Map<String, dynamic> content;
-  const IndexMainContent3(this.content, {Key key}) : super(key: key);
+  const IndexMainContent3(this.content, {Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {

lib/widgets/general/navigationbar_logo.dart

@@ -4,7 +4,7 @@ import 'package:flutter/material.dart';
 import '../../constants.dart';
 
 class NavigationBarLogo extends StatelessWidget {
-  const NavigationBarLogo({Key key}) : super(key: key);
+  const NavigationBarLogo({Key? key}) : super(key: key);
 
   static const IconData logoData = IconData(
       Constants.FONT_WISETRONIC,

lib/widgets/general/payment_verification_code_dialog.dart

@@ -12,7 +12,7 @@ typedef OnCodeVerified();
 typedef OnCancel();
 
 class PaymentVerificationCodeDialog extends StatefulWidget {
-  final Key key;
+  final Key? key;
   final User user;
   final OnCodeVerified onCodeVerified;
   final OnCancel onCancel;

lib/widgets/general/read_more_text.dart

@@ -9,7 +9,7 @@ enum TrimMode {
 class ReadMoreText extends StatefulWidget {
   const ReadMoreText(
       this.data, {
-        Key key,
+        Key? key,
         this.trimExpandedText = ' read less',
         this.trimCollapsedText = ' ...read more',
         this.colorClickableText,

lib/widgets/general/sliding_up_panel.dart

@@ -164,7 +164,7 @@ class SlidingUpPanel extends StatefulWidget {
   final PanelState defaultPanelState;
 
   SlidingUpPanel({
-    Key key,
+    Key? key,
     this.panel,
     this.panelBuilder,
     this.body,