Property myWidgetClassName does not exist on type JSX.IntrinsicElements.
Creating a custom pluggable widget causes this error: Property 'myWidgetClassName' does not exist on type 'JSX.IntrinsicElements'.
The reason is this:
Basically, make sure all types start with a capital letter. Don’t name your class to follow typically JS naming conventions for the HTML tags. This is a TypeScript specific issue.