Fix runtime error with fast-refresh

Changelog.md

@@ -7,7 +7,11 @@
 - Calling `startPolling` or `stopPolling` after a component has unmounted is now a no-op (instead of throwing an exception). Polling is automatically stopped when a component is unmounted, so it doesn't need to be called manually. <br/>
   [@hwillson](https://github.com/hwillson) in [#3485](https://github.com/apollographql/react-apollo/pull/3485)
 - Allow `ignoreResults` to be controlled through `graphql` and `withMutation` options. <br/>
-  [@tim-stasse](https://github.com/tim-stasse) in [#3431](https://github.com/apollographql/react-apollo/pull/3431) 
+  [@tim-stasse](https://github.com/tim-stasse) in [#3431](https://github.com/apollographql/react-apollo/pull/3431)
+- Be a bit more defensive when it comes to accessing the internal
+  `ObservableQuery` instance, to avoid attempting to use it after a component
+  has unmounted. <br/>
+  [@jfrolich](https://github.com/jfrolich) in [#3490](https://github.com/apollographql/react-apollo/pull/3490)
 
 ### Bug Fixes
 

packages/hooks/src/data/QueryData.ts

@@ -418,11 +418,10 @@ export class QueryData<TData, TVariables> extends OperationData {
   }
 
   private handleErrorOrCompleted() {
-    const {
-      data,
-      loading,
-      error
-    } = this.currentObservable.query!.getCurrentResult();
+    const obsQuery = this.currentObservable.query;
+    if (!obsQuery) return;
+
+    const { data, loading, error } = obsQuery.getCurrentResult();
 
     if (!loading) {
       const { query, variables, onCompleted, onError } = this.getOptions();