[BUGFIX] fix the error message in the data loader. (#509)

* fix.

* Change to required=True

* Update description

* Change back

python/dgl/data/__init__.py

@@ -15,7 +15,7 @@ from .gindt import GINDataset
 
 def register_data_args(parser):
     parser.add_argument("--dataset", type=str, required=False,
-            help="The input dataset.")
+            help="The input dataset. Can be cora, citeseer, pubmed, syn(synthetic dataset) or reddit")
     citegrh.register_args(parser)
 
 def load_data(args):
@@ -27,7 +27,7 @@ def load_data(args):
         return citegrh.load_pubmed()
     elif args.dataset == 'syn':
         return citegrh.load_synthetic(args)
-    elif args.dataset.startswith('reddit'):
+    elif args.dataset is not None and args.dataset.startswith('reddit'):
         return RedditDataset(self_loop=('self-loop' in args.dataset))
     else:
         raise ValueError('Unknown dataset: {}'.format(args.dataset))