add safety guard to serializers & fix importing uncorrect serializer

src/api/serializers/post.ts

@@ -57,6 +57,8 @@ const self = async (
 		_post = deepcopy(post);
 	}
 
+	if (!_post) throw 'invalid post arg.';	
+
 	const id = _post._id;
 
 	// Rename _id to id

src/api/serializers/user.ts

@@ -56,6 +56,8 @@ export default (
 		_user = deepcopy(user);
 	}
 
+	if (!_user) return reject('invalid user arg.');
+
 	// Me
 	const meId: mongo.ObjectID = me
 		? mongo.ObjectID.prototype.isPrototypeOf(me)