Start to fix null safety migration errors

lib/ui/widgets/dialogs/auto_sized_dialog_content_widget.dart

@@ -7,8 +7,8 @@ class AutoSizeDialogContentWidget extends StatelessWidget {
   final Widget child;
 
   const AutoSizeDialogContentWidget({
-    Key key,
-    @required this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   @override

lib/ui/widgets/dialogs/cancel_dialog_button.dart

@@ -10,7 +10,7 @@ class CancelDialogButton extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialButton(
       onPressed: () => Navigator.of(context).pop(),
-      child: Text(tr("Cancel").toUpperCase()),
+      child: Text(tr("Cancel")!.toUpperCase()),
       textColor: Colors.red,
     );
   }

lib/ui/widgets/dialogs/confirm_dialog_button.dart

@@ -10,9 +10,9 @@ class ConfirmDialogButton<T> extends StatelessWidget {
   final T value;
 
   const ConfirmDialogButton({
-    Key key,
+    Key? key,
     this.enabled = true,
-    @required this.value,
+    required this.value,
   })  : assert(enabled != null),
         super(key: key);
 
@@ -20,7 +20,7 @@ class ConfirmDialogButton<T> extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialButton(
       onPressed: enabled ? () => Navigator.of(context).pop(value) : null,
-      child: Text(tr("Confirm").toUpperCase()),
+      child: Text(tr("Confirm")!.toUpperCase()),
       textColor: Colors.green,
     );
   }