Loop occasionally fails to start if LND chain notifier RPC is still in the process of starting

[rockstardev]

I've added Loop to BtcPayServer LND container and am starting it after unlocking the wallet. Occasionally starting Loop will fail. Here is relevant log:

2021-04-25 18:56:09.637 [INF] LOOP: Wait for first block ntfn
2021-04-25 18:56:09.637 [INF] LOOPD: Waiting for updates
2021-04-25 18:56:09.637 [INF] LOOPD: Starting liquidity manager
2021-04-25 18:56:09.638 [INF] LOOPD: REST proxy listening on [::]:8081
2021-04-25 18:56:09.639 [ERR] RPCS: [/chainrpc.ChainNotifier/RegisterBlockEpochNtfn]: chain notifier RPC is still in the process of starting
2021-04-25 18:56:09.640 [ERR] LOOP: Swap client terminating: rpc error: code = Unknown desc = chain notifier RPC is still in the process of starting
2021-04-25 18:56:09.640 [INF] LOOP: Swap client terminated
2021-04-25 18:56:09.640 [INF] LOOPD: Swap client stopped
2021-04-25 18:56:09.640 [ERR] LOOPD: Runtime error in daemon, shutting down: rpc error: code = Unknown desc = chain notifier RPC is still in the process of starting
2021-04-25 18:56:09.640 [INF] LOOPD: Stopping gRPC server
2021-04-25 18:56:09.640 [INF] LOOPD: Liquidity manager stopped
2021-04-25 18:56:09.641 [INF] LOOPD: Stopping REST server
2021-04-25 18:56:09.641 [INF] LOOPD: Daemon exited
loopd exited with an error: rpc error: code = Unknown desc = chain notifier RPC is still in the process of starting

To reproduce you would just start LND, unlock wallet and then immediately start Loop. The reason why I'm getting this problem occasionally is that I've added 15 second wait between unlocking LND wallet and starting Loop.

Here is the script that starts Loop:
https://github.com/btcpayserver/lnd/blob/basedon-v0.12.1-beta-15s/docker-initunlocklnd.sh#L88

If needed I can provide ssh to server that has LND+Loop docker container running for more detailed debugging.