fix(dot/network): re-add nil mutex check for disconnected peer (#2408)

dot/network/notifications.go

@@ -284,6 +284,8 @@ func (s *Service) sendData(peer peer.ID, hs Handshake, info *notificationsProtoc
 	}, peer)
 }
 
+var errPeerDisconnected = errors.New("peer disconnected")
+
 func (s *Service) sendHandshake(peer peer.ID, hs Handshake, info *notificationsProtocol) (libp2pnetwork.Stream, error) {
 	// multiple processes could each call this upcoming section, opening multiple streams and
 	// sending multiple handshakes. thus, we need to have a per-peer and per-protocol lock
@@ -293,6 +295,12 @@ func (s *Service) sendHandshake(peer peer.ID, hs Handshake, info *notificationsP
 	// so we cannot have a method on peersData to lock and unlock the mutex
 	// from the map
 	peerMutex := info.peersData.getMutex(peer)
+	if peerMutex == nil {
+		// Note: the only place the mutex is deleted is when the peer disconnects.
+		// If it doesn't exist, the peer never connected either.
+		return nil, fmt.Errorf("%w: peer id %s", errPeerDisconnected, peer)
+	}
+
 	peerMutex.Lock()
 	defer peerMutex.Unlock()