Kilian responded to this on Jaspr's Discord. Also, I think this issue was encountered during the live demo with Craig on observable flutter and doing 'jaspr clean' solved it.

Kilian's response: Thats not a user error, but a weird build_runner issue with removing @client components and hotreloading. Doing 'jaspr clean' and restarting 'jaspr serve' should make the error go away.