fix: error when server throws and we are also querying for a local field

.changeset/small-lemons-grow.md

@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+Fix error when server returns an error and we are also querying for a local field

src/__tests__/local-state/general.ts

@@ -1196,4 +1196,45 @@ describe('Combining client and server state/operations', () => {
       },
     });
   });
+
+  itAsync('should handle server errors when field is required', (resolve, reject) => {
+    const query = gql`
+      query GetUser {
+        user {
+          firstName @client
+          lastName
+        }
+      }
+    `;
+
+    const cache = new InMemoryCache();
+    const link = new ApolloLink(operation => {
+      expect(operation.operationName).toBe('GetUser');
+      return Observable.of({
+        data: null,
+        errors: [new GraphQLError("something went wrong", {
+          extensions: {
+            code: "INTERNAL_SERVER_ERROR"
+          },
+          path: ["user"]
+        })]
+      });
+    });
+
+    const client = new ApolloClient({
+      cache,
+      link,
+      resolvers: {},
+    });
+
+    client.watchQuery({ query }).subscribe({
+      error(error) {
+        expect(error.message).toEqual("something went wrong");
+        resolve();
+      },
+      next() {
+        reject();
+      },
+    });
+  });
 });

src/core/LocalState.ts

@@ -353,6 +353,10 @@ export class LocalState<TCacheShape> {
     rootValue: any,
     execContext: ExecContext,
   ): Promise<any> {
+    if (!rootValue) {
+      return null;
+    }
+
     const { variables } = execContext;
     const fieldName = field.name.value;
     const aliasedFieldName = resultKeyNameFromField(field);