Bugfix in sanity check: Prevent null-dereference

lib/src/db/db.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of gcloud.db;
 
 /**
  * A function definition for transactional functions.
  *
  * The function will be given a [Transaction] object which can be used to make
@@ skipping 297 matching lines @@
     // TODO(#26): There is only one case where `partition` is not redundant
     // Namely if `ancestorKey == null` and `partition != null`. We could
     // say we get rid of `partition` and enforce `ancestorKey` to
     // be `Partition.emptyKey`?
     if (partition == null) {
       if (ancestorKey != null) {
         partition = ancestorKey.partition;
       } else {
         partition = defaultPartition;
       }
-    } else if (partition != ancestorKey.partition) {
+    } else if (ancestorKey != null && partition != ancestorKey.partition) {
       throw new ArgumentError(
           'Ancestor queries must have the same partition in the ancestor key '
           'as the partition where the query executes in.');
     }
     return new Query(this,
                      kind,
                      partition: partition,
                      ancestorKey: ancestorKey);
   }
 
@@ skipping 68 matching lines @@
 
 Future<List<Model>> _lookupHelper(
     DatastoreDB db, List<Key> keys,
     {datastore.Transaction datastoreTransaction}) {
   var entityKeys = keys.map(db.modelDB.toDatastoreKey).toList();
   return db.datastore.lookup(entityKeys, transaction: datastoreTransaction)
       .then((List<datastore.Entity> entities) {
     return entities.map(db.modelDB.fromDatastoreEntity).toList();
   });
 }