Fix a few notices

lib/ui/widgets/conversation_image_widget.dart

@@ -24,9 +24,7 @@ class ConversationImageWidget extends StatelessWidget {
     this.group,
     this.size = 30,
     this.noUserImage,
-  })  : assert(conversation != null),
-        assert(users != null),
-        assert(size > 0),
+  })  : assert(size > 0),
         super(key: key);
 
   @override

lib/ui/widgets/custom_app_bar_size.dart

@@ -10,9 +10,7 @@ class AppBarWrapper extends StatelessWidget implements PreferredSizeWidget {
   final double height;
 
   const AppBarWrapper({required this.height, required this.appBar})
-      : assert(height != null),
-        assert(appBar != null),
-        super();
+      : super();
 
   @override
   Widget build(BuildContext context) => appBar;