Update parse_relation.dart

Author: RodrigoSMarques

packages/dart/lib/src/objects/parse_relation.dart

@@ -3,6 +3,9 @@ part of flutter_parse_sdk;
 // ignore_for_file: always_specify_types
 class ParseRelation<T extends ParseObject> {
   ParseRelation({required ParseObject parent, required String key}) {
+    if (!parent.containsKey(key)) {
+      throw 'Invalid Relation key name';
+    }
     _targetClass = parent.get<ParseRelation>(key)!.getTargetClass;
     _parent = parent;
     _key = key;