Fixed NPE on getting null item of collection.

Fixed NullPointerException on getting null item of collection.

Author: Maros-1984

src/main/java/de/danielbechler/diff/accessor/CollectionItemAccessor.java

@@ -59,7 +59,7 @@ public Object get(final Object target)
 		}
 		for (final Object item : targetCollection)
 		{
-			if (item.equals(referenceItem))
+			if (item != null && item.equals(referenceItem))
 			{
 				return item;
 			}

src/test/java/de/danielbechler/diff/integration/DeepDiffingCollectionItemChangeIntegrationTest.java

@@ -51,4 +51,10 @@ public void test_returns_full_property_graph_of_added_collection_items()
 										   .withMapKey("foo")
 										   .withPropertyName("value")).hasState(Node.State.ADDED);
 	}
+
+	@Test
+	public void test_collection_with_null_item()
+	{
+		ObjectDifferFactory.getInstance().compare(Arrays.asList((String)null), Arrays.asList("foobar"));
+	}
 }