Fix unread conversations drop down

lib/lists/unread_conversations_list.dart

@@ -8,7 +8,7 @@ import 'package:comunic/models/unread_conversation.dart';
 class UnreadConversationsList extends AbstractList<UnreadConversation> {
   /// Get the ID of the users included in this list
   Set<int> get usersID {
-    final set = Set();
+    final set = Set<int>();
     forEach((element) => set.addAll(element.message.usersID));
     return set;
   }