Fix crash in unsubscribeFromQuery

See #255.

Author: glasser

src/graphql.tsx

@@ -416,9 +416,11 @@ export default function graphql(
       }
 
       unsubscribeFromQuery() {
-        if ((this.querySubscription as Subscription).unsubscribe) {
+        if (this.querySubscription &&
+            (this.querySubscription as Subscription).unsubscribe) {
           (this.querySubscription as Subscription).unsubscribe();
           delete this.queryObservable;
+          delete this.querySubscription;
         }
       }