TypeError: Cannot read property 'readyState' of null

[huan]

01:28:54 VERB IoService onMessage({data: {"name":"heartbeat","payload":{"uuid":"9ea9f1e8-478e-47af-8ca8-7e65cac881e3","data":"heartbeat@browser"}}})
angular:118 01:28:54 SILL IoService init() event.subscribe({name:heartbeat})
angular:576 01:28:54 SILL WechatyComponent onIo#505(heartbeat)
app.component.ts:32 01:28:54 INFO AppComponent onEvent(heartbeat, [object Object])
angular:421 01:29:09 VERB IoService onMessage({data: {"name":"heartbeat","payload":{"uuid":"9ea9f1e8-478e-47af-8ca8-7e65cac881e3","data":"heartbeat@browser"}}})
angular:118 01:29:09 SILL IoService init() event.subscribe({name:heartbeat})
angular:576 01:29:09 SILL WechatyComponent onIo#506(heartbeat)
app.component.ts:32 01:29:09 INFO AppComponent onEvent(heartbeat, [object Object])
angular:449 01:30:04 VERB IoService socketOnClose({code:1006, reason:, returnValue:true})
errors.js:48 ERROR TypeError: Cannot read property 'readyState' of null
    at IoService.socketUpdateState (VM196 angular:408)
    at IoService.socketOnClose (VM196 angular:450)
    at WebSocket.wrapFn [as __zone_symbol___onclose] (VM159 zone.js:1393)
    at ZoneDelegate.invokeTask (VM159 zone.js:398)
    at Object.onInvokeTask (VM192 core.umd.js:4656)
    at ZoneDelegate.invokeTask (VM159 zone.js:397)
    at Zone.runTask (VM159 zone.js:165)
    at WebSocket.ZoneTask.invoke (VM159 zone.js:460)