FileStore.IndexBlockhash return null when index out of range

CHANGES.md

@@ -88,9 +88,13 @@ To be released.
  -  Fixed a bug that `TurnClient` had not stopped properly.  [[#287]]
  -  Fixed a bug that `TurnClient` had been trying to use an already closed
     connection. [[#303], [#308]]
+ -  Fixed a bug that `KeyNotFoundException` had been thrown instead of
+    `ArgumentOutOfRangeException` when `Blockchain<T>[int]` called while the
+    index of a block that does not exist locally.  [[#208], [#317]]
 
 
 [LiteDB]: https://www.litedb.org/
+[#208]: https://github.com/planetarium/libplanet/issues/208
 [#267]: https://github.com/planetarium/libplanet/issues/267
 [#269]: https://github.com/planetarium/libplanet/pull/269
 [#270]: https://github.com/planetarium/libplanet/pull/270
@@ -112,6 +116,7 @@ To be released.
 [#308]: https://github.com/planetarium/libplanet/pull/308
 [#309]: https://github.com/planetarium/libplanet/issues/309
 [#310]: https://github.com/planetarium/libplanet/pull/310
+[#317]: https://github.com/planetarium/libplanet/pull/317
 
 
 Version 0.3.0

Libplanet.Tests/Store/StoreTest.cs

@@ -470,5 +470,14 @@ public void TxNonce()
                 Fx.Store.ListTxNonces(Fx.StoreNamespace).ToDictionary(p => p.Key, p => p.Value)
             );
         }
+
+        [Fact]
+        public void IndexBlockHashReturnNull()
+        {
+            Fx.Store.PutBlock(Fx.Block1);
+            Fx.Store.AppendIndex(Fx.StoreNamespace, Fx.Block1.Hash);
+            Assert.Equal(1, Fx.Store.CountIndex(Fx.StoreNamespace));
+            Assert.Null(Fx.Store.IndexBlockHash(Fx.StoreNamespace, 2));
+        }
     }
 }

Libplanet/Store/FileStore.cs

@@ -362,6 +362,10 @@ long index
                     return null;
                 }
             }
+            else if (CountIndex(@namespace) < index)
+            {
+                return null;
+            }
 
             if (!File.Exists(GetIndexPath(@namespace)))
             {