SharePoint Framework App-extension error

I'm facing an issue following the dev article for creating an App-extension. Following this article, the step where you're asked to gulp-serve and append an existing list with the following code.

?loadSPFX=true&debugManifestsFile=https://localhost:4321/temp/manifests.js&customActions={"d03ae0c2-bbbf-4cf5-9ff7-0986904553da":{"location":"ClientSideExtension.ApplicationCustomizer","properties":{"testMessage":"Hello as property!"}}}

I have replaced this code with my own ID an tenant name, resulting in the following URL. 

https://mytenant.sharepoint.com/sites/dev/Lists/test/AllItems.aspx?loadSPFX=true&debugManifestsFile=https://localhost:4321/temp/manifests.js&customActions={"7c36083f-4b95-43a3-ad8c-d8ed500043a2":{"location":"ClientSideExtension.ApplicationCustomizer","properties":{"testMessage":"Hello as property!"}}}

Browsing the URL results the console error below. Does anyone have an idea what might cause this problem?