Description of problem:

When the user close a quickstart sidebar with the x-icon in the top right corner, the browser log shows an error. (The UI doesn't crash.)

(In React dev mode the UI shows an fullscreen error in Firefox but not in Chrome. The error log is shown in both browsers.)

Prerequisites (if any, like setup, operators/versions):

None

Steps to Reproduce

1. Open a quick start
2. Close the quick start with the top right corner x-icon

Actual results:

Got an error in the browser console:

unhandled error: TypeError: error is null ./public/components/app.jsx/</</window.onerror@http://localhost:9000/static/main-f871bd8f61069fae6854.js:96737:9
OnErrorEventHandlerNonNull*./public/components/app.jsx/</<@http://localhost:9000/static/main-f871bd8f61069fae6854.js:96734:3
setReady@http://localhost:9000/static/main-f871bd8f61069fae6854.js:152069:12
connectionCallback@http://localhost:9000/static/main-f871bd8f61069fae6854.js:152090:18
./node_modules/subscriptions-transport-ws/dist/client.js/</SubscriptionClient.prototype.processReceivedData@http://localhost:9000/static/vendors~main-8e5206883a5fb4d94926.js:343855:26

Expected results:

No error or warning in the browser console

Reproducibility (Always/Intermittent/Only Once):

Always

Build Details:

4.9 master (tested with commit 987e9de1b3803b86f594a62e0c19c547411e84d6)

Additional info:

None