Can't start WebSocket transport

[stdout] [Rivet] Starting WebSocket server: 127.0.0.1:7777
[stderr] ERROR: Condition "get_connection_status() != CONNECTION_DISCONNECTED" is true. Returning: ERR_ALREADY_IN_USE
[stderr]    at: create_server (modules/websocket/websocket_multiplayer_peer.cpp:181)
[stderr] ERROR: Failed to create WebSocket server
[stderr]    at: (core/core_bind.cpp:254)

const PORT = 7777
const HOST = "127.0.0.1"
var _server = WebSocketMultiplayerPeer.new()

func _ready():
    var err = _server.create_server(PORT, HOST)
    if err != OK:
        print("Unable to start server")
        set_process(false)
    else:
        print("Server started")