Prevent using an unregistered custom element for a route Component

[OlliTietavainenVaadin]

If you happen to define your routes with a component that hasn't been added to the custom element registry (perhaps due to a missing import), you will end up with a route that fails with no immediately obvious reason. Router could either warn or fail in this scenario. This should not prevent you from lazy-loading routes, as you use the action property (instead of component) to define a function for dynamically loaded routes.