Fix bug in collection creation

lib/models/magic_metadata.dart

@@ -67,7 +67,7 @@ class CollectionMagicMetadata {
   // 1 -> DEFAULT_HIDDEN COLLECTION for files hidden individually
   int? subType;
 
-  CollectionMagicMetadata({required this.visibility, int? subType});
+  CollectionMagicMetadata({required this.visibility, this.subType});
 
   Map<String, dynamic> toJson() {
     final result = {magicKeyVisibility: visibility};

lib/services/hidden_service.dart

@@ -100,6 +100,7 @@ extension HiddenService on CollectionsService {
       visibility: visibilityHidden,
       subType: subTypeDefaultHidden,
     ).toJson();
+    assert(jsonToUpdate.length == 2, "metadata should have two keys");
     final encryptedMMd = await CryptoUtil.encryptChaCha(
       utf8.encode(jsonEncode(jsonToUpdate)) as Uint8List,
       key,