从任何类转换到另一个类在Kotlin中返回null

我有一个动态Json,我正试图这样解析它

obs.subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .unsubscribeOn(Schedulers.io())
    .subscribe(object: Observer <Response<Any>> {
        override fun onComplete() {}

        override fun onSubscribe(d: Disposable) {}

        override fun onNext(t: Response<Any>) {
            if (t.code() != 400) {
                val registerResponse = t.body() as RegisterResponse?
                    //this is null
                    println("value of register resp " + registerResponse?.status)
                liveData.value = t.body() as RegisterResponse ?

            } else {
                val validationError = t.body() as ValidationError?
                    //this is also null
                    println("error resp " + validationError?.message?.password?.get(0))
                error.value = t.body() as ValidationError?
            }
        }

        override fun onError(e: Throwable) {}

})