NPE in WebSocket client subscriptions

[cthulhu-rider]

caught suddenly while debugging nspcc-dev/neofs-node#2444, don’t have a special context. I subscribe to new blocks and notary requests

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x7bc119]

goroutine 14 [running]:
github.com/nspcc-dev/neo-go/pkg/rpcclient.(*WSClient).notifySubscribers(0xc000224480, {0x16?, {0xa11180?, 0xc000283ea0?}})
	/home/ll/go/pkg/mod/github.com/nspcc-dev/neo-go@v0.101.1/pkg/rpcclient/wsclient.go:599 +0x299
github.com/nspcc-dev/neo-go/pkg/rpcclient.(*WSClient).wsReader(0xc000224480)
	/home/ll/go/pkg/mod/github.com/nspcc-dev/neo-go@v0.101.1/pkg/rpcclient/wsclient.go:513 +0x545
created by github.com/nspcc-dev/neo-go/pkg/rpcclient.NewWS
	/home/ll/go/pkg/mod/github.com/nspcc-dev/neo-go@v0.101.1/pkg/rpcclient/wsclient.go:422 +0x638

may be i violated some strict requirement on my side, but didn't find it after a quick look

[AnnaShaleva]

The issue is caused by the fact that subscriptions map of WS client isn't in sync with receivers map (although it always must be). And currently I can't find where the bug is and also can't reproduce the panic. It's possible that concurrency issue occurs between subsequent event receiving and unsubscription.