There is a missing nil check in AddBlockHeader handler which will cause the zetacored to panic, but seems to recover or at least I was not able to generate a crash, which is why I submit this only as Low.
While Observers that broadcast those AddBlockHeader message are being verified, such that an unknown chain is not possible, there is always the risk of a malicious Observer (or malicious user that have access to the node machine which could use the CLI to broadcast) to broadcast a malicious AddBlockHeader message which would include an unknown chain and trigger this panic.
I would recommend to add the missing nil check.
https://github.com/code-423n4/2023-11-zetachain/blob/main/repos/node/x/observer/keeper/msg_server_add_block_header.go#L18-L21
