After Update to 0.20 from 0.19 the error Cannot assign to read only property 'defaults' of object '[object Object]' occurs

The release says that there a breaking change. Try that and see if the problem goes away. I got the same error but haven't tried the fix yet. https://github.com/artf/grapesjs/releases/tag/v0.20.1

Yes, as mentioned in the release note you have to switch from the old legacy API for custom component registration.