Start to fix null safety migration errors

lib/ui/widgets/tour/account_image_tour_pane.dart

@@ -9,11 +9,11 @@ class AccountImageTourPane extends PresentationPane {
   final Function(BuildContext) onUpdated;
 
   AccountImageTourPane({
-    @required User user,
-    @required this.onUpdated,
+    required User user,
+    required this.onUpdated,
   }) : super(
             iconWidget: AccountImageWidget(user: user, width: 50),
-            title: tr("Account image"),
+            title: tr("Account image")!,
             text:
                 "${tr("Account images allow to quickly recognize people.")}\n\n${tr("You can decide to define one now!")}",
             actionTitle: tr("Upload an account image"),

lib/ui/widgets/tour/first_pane.dart

@@ -8,10 +8,10 @@ import 'package:flutter/material.dart';
 /// @author Pierre Hubert
 
 class FirstTourPane extends StatelessWidget {
-  final String msgOne;
-  final String msgTwo;
+  final String? msgOne;
+  final String? msgTwo;
 
-  const FirstTourPane({Key key, this.msgOne, this.msgTwo}) : super(key: key);
+  const FirstTourPane({Key? key, this.msgOne, this.msgTwo}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {

lib/ui/widgets/tour/fixed_tour_size_text_area.dart

@@ -4,13 +4,13 @@
 import 'package:flutter/material.dart';
 
 class FixedTourSizeTextArea extends StatelessWidget {
-  final String text;
+  final String? text;
 
   const FixedTourSizeTextArea(this.text);
 
   @override
   Widget build(BuildContext context) => ConstrainedBox(
     constraints: BoxConstraints(maxWidth: 300),
-    child: Text(text, textAlign: TextAlign.justify),
+    child: Text(text!, textAlign: TextAlign.justify),
   );
 }

lib/ui/widgets/tour/last_pane.dart

@@ -10,7 +10,7 @@ class LastTourPane extends StatelessWidget {
   Widget build(BuildContext context) {
     return Center(
       child: Text(
-        tr("The application is yours"),
+        tr("The application is yours")!,
         style: TextStyle(fontSize: 25),
       ),
     );

lib/ui/widgets/tour/presentation_pane.dart

@@ -5,22 +5,22 @@ import 'package:comunic/ui/widgets/tour/fixed_tour_size_text_area.dart';
 import 'package:flutter/material.dart';
 
 class PresentationPane extends StatelessWidget {
-  final IconData icon;
-  final Widget iconWidget;
+  final IconData? icon;
+  final Widget? iconWidget;
   final String title;
-  final String text;
-  final Function(BuildContext) child;
-  final String actionTitle;
-  final Function(BuildContext) onActionTap;
+  final String? text;
+  final Function(BuildContext)? child;
+  final String? actionTitle;
+  final Function(BuildContext)? onActionTap;
   final bool canGoNext;
-  final Future<bool> Function(BuildContext) onTapNext;
+  final Future<bool> Function(BuildContext)? onTapNext;
   final bool visible;
 
   const PresentationPane({
-    Key key,
+    Key? key,
     this.icon,
     this.iconWidget,
-    @required this.title,
+    required this.title,
     this.text,
     this.child,
     this.actionTitle,
@@ -41,7 +41,7 @@ class PresentationPane extends StatelessWidget {
       return Column(
         children: <Widget>[
           Spacer(flex: 3),
-          icon != null ? Icon(icon, color: Colors.white, size: 50) : iconWidget,
+          icon != null ? Icon(icon, color: Colors.white, size: 50) : iconWidget!,
           Spacer(flex: 1),
           Text(
             title,
@@ -52,9 +52,9 @@ class PresentationPane extends StatelessWidget {
           Spacer(flex: 1),
           _hasAction
               ? OutlinedButton(
-                  onPressed: () => onActionTap(context),
+                  onPressed: () => onActionTap!(context),
                   child: Text(
-                    actionTitle,
+                    actionTitle!,
                     style: TextStyle(color: Colors.white),
                   ),
                 )
@@ -72,7 +72,7 @@ class PresentationPane extends StatelessWidget {
     return Column(
       children: <Widget>[
         Spacer(flex: 1),
-        icon != null ? Icon(icon, color: Colors.white, size: 50) : iconWidget,
+        icon != null ? Icon(icon, color: Colors.white, size: 50) : iconWidget!,
         Spacer(flex: 1),
         Text(
           title,
@@ -81,7 +81,7 @@ class PresentationPane extends StatelessWidget {
         Spacer(flex: 1),
         ConstrainedBox(
             constraints: BoxConstraints(maxHeight: 300),
-            child: SingleChildScrollView(child: child(context))),
+            child: SingleChildScrollView(child: child!(context))),
         Spacer(flex: 1),
       ],
     );

lib/ui/widgets/tour/tour_notifications_pane.dart

@@ -11,24 +11,24 @@ import 'package:flutter/material.dart';
 
 class TourNotificationsPane extends PresentationPane {
   TourNotificationsPane({
-    Key key,
-    @required
+    Key? key,
+    required
         GlobalKey<PushNotificationsConfigurationWidgetState>
             pushNotificationsKey,
-    @required Function() onConfigured,
-    @required Function() onChanged,
-    @required bool visible,
+    required Function() onConfigured,
+    required Function() onChanged,
+    required bool visible,
   })  : assert(pushNotificationsKey != null),
         super(
           icon: Icons.notifications,
-          title: tr("Push notifications"),
+          title: tr("Push notifications")!,
           child: (c) => PushNotificationsConfigurationWidget(
             key: pushNotificationsKey,
             onConfigured: onConfigured,
             onChanged: onChanged,
           ),
           canGoNext: pushNotificationsKey?.currentState?.canSubmit ?? false,
-          onTapNext: (c) => pushNotificationsKey.currentState.submit(),
+          onTapNext: (c) => pushNotificationsKey.currentState!.submit(),
           visible: visible,
         );
 }