throw error when worker ref not found

src/main/connection_helper.ts

@@ -67,6 +67,9 @@ export class ConnectionHelper {
     if (workerRef) {
       this.queryManager = new workerRef.QueryManager(this.processFinishedQuery_.bind(this));
     }
+    else {
+      throw new Error('JsStoreWorker not found. Either pass web worker or inject JsStore Worker.');
+    }
   }
 
   private onMessageFromWorker_(msg) {