Debug-only crash when locking wallet immediately after restore

[StephenHeaps]

Description

On debug builds, a NOTREACHED(); is hit after restoring a wallet and immediately locking the wallet.
Hit on iOS, but I can reproduce using 1.59.x, 1.60.x and master of brave-core repo. Theory in Miscellaneous Information below.

Steps to Reproduce

1. Restore a wallet
2. Immediately lock the wallet
3. Crashes in Account Discovery service on a NOTREACHED();: https://github.com/brave/brave-core/blob/master/components/brave_wallet/browser/account_discovery_manager.cc#L74

Actual result:

Crashes

Expected result:

Does not crash

Reproduces how often:

Easily reproduced (on iOS)

Brave version (brave://version info)

1.59+

Version/Channel Information:

• Can you reproduce this issue with the current release?
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the nightly channel?

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

I suspect this is a race condition but I've not fully debugged it.

NOTREACHED is hit in account_discovery_manager.cc at L74:
https://github.com/brave/brave-core/blob/fffea2b96c7bc255a66101166461cfe7b1d81fdb/components/brave_wallet/browser/account_discovery_manager.cc#L74

This can be hit when KeyringService::GetDiscoveryAddress returns null:
https://github.com/brave/brave-core/blob/fffea2b96c7bc255a66101166461cfe7b1d81fdb/components/brave_wallet/browser/keyring_service.cc#L1753-L1758

KeyringService::GetDiscoveryAddress can return null when KeyringService::GetHDKeyringById returns null:
https://github.com/brave/brave-core/blob/fffea2b96c7bc255a66101166461cfe7b1d81fdb/components/brave_wallet/browser/keyring_service.cc#L1429-L1434

KeyringService::GetHDKeyringById returns null when _keyrings does not contain the keyring for the given id. _keyrings is cleared when wallet is locked:
https://github.com/brave/brave-core/blob/fffea2b96c7bc255a66101166461cfe7b1d81fdb/components/brave_wallet/browser/keyring_service.cc#L1930