fix: added check for observer (#735)

Author: simon-q

packages/aws-appsync-subscription-link/src/realtime-subscription-handshake-link.ts

@@ -755,16 +755,21 @@ export class AppSyncRealTimeSubscriptionHandshakeLink extends ApolloLink {
         subscriptionState
       });
 
-      observer.error({
-        errors: [
-          {
-            ...new GraphQLError(`Connection failed: ${JSON.stringify(payload)}`)
-          }
-        ]
-      });
       clearTimeout(startAckTimeoutId);
 
-      observer.complete();
+      if (observer) {
+        observer.error({
+          errors: [
+            {
+              ...new GraphQLError(`Connection failed: ${JSON.stringify(payload)}`)
+            }
+          ]
+        });
+        observer.complete();
+      } else {
+        logger(`observer not found for id: ${id}`);
+      }
+      
       if (typeof subscriptionFailedCallback === "function") {
         subscriptionFailedCallback();
       }