[v22.7.0] Crash when voluntarily exiting a validator

[jclapis]

Describe the bug
I'm using v22.7.0 on Ropsten. I attempted to submit a voluntary exit message for one of my validators, and Nimbus crashed with the following logs:

eth2_1           | DBG 2022-08-15 08:00:44.664+00:00 Received request                           topics="beacnde" peer=172.18.0.7:42214 meth=POST uri=/eth/v1/beacon/pool/voluntary_exits
eth2_1           | DBG 2022-08-15 08:00:44.665+00:00 Serving API request                        topics="beacnde" peer=172.18.0.7:42214 meth=POST uri=/eth/v1/beacon/pool/voluntary_exits path_params=[] query_params=[] content_body="(application/json, 265 bytes)"
eth2_1           | DBG 2022-08-15 08:00:44.666+00:00 Voluntary exit received                    topics="gossip_eth2" signedVoluntaryExit="(message: (epoch: 17259, validator_index: 101108), signature: \"b552c7c6\")"
eth2_1           | [[reraised from:
eth2_1           | ]]
eth2_1           | [[reraised from:
eth2_1           | ]]
eth2_1           | [[reraised from:
eth2_1           | ]]
eth2_1           | [[reraised from:
eth2_1           | ]]
eth2_1           | Error: unhandled exception: Trying to access error when value is set [ResultDefect]

This occurs whenever I try to exit any of my validators.

To Reproduce
Steps to reproduce the behavior:

• Create a validator
• Wait until it's eligible to exit
• Attempt to send a voluntary exit message
• Observe the crash

1. Platform details (OS, architecture): Ubuntu 20.04 arm64
2. Branch/commit used: v22.7.0
3. Commands being executed: standard Beacon Node execution
4. Relevant log lines: see above

Additional context
@cheatfate may have identified the cause here: https://github.com/status-im/nimbus-eth2/blob/unstable/beacon_chain/validators/message_router.nim#L432

See this Discord thread for more info.