Why am I getting a 'closed' exception when trying to write OkHttp response to .json file in Kotlin?

I am trying to make a Currency Conversion App in Kotlin and have a problem with updating my values. The connection works fine but I cannnot write it into an .json file.
Here is my Code:

        val request = Request.Builder().url(url).header("apikey", key).build()

        val client = OkHttpClient()
        client.newCall(request).enqueue(object : Callback {
            override fun onFailure(call: Call, e: IOException) {}
            override fun onResponse(call: Call, response: Response) {
                println(response.body()?.string())
                val date = LocalDate.now()
                val osw = OutputStreamWriter(openFileOutput("$date.json", MODE_PRIVATE))
                osw.write(response.body()?.string())
                println("finished")
            }
        })
    }

Here is the Error: 
E/AndroidRuntime: FATAL EXCEPTION: OkHttp Dispatcher
    Process: com.example.appdevelopment_assignment_7, PID: 6787
    java.lang.IllegalStateException: closed
        at okio.RealBufferedSource.rangeEquals(RealBufferedSource.java:398)
        at okio.RealBufferedSource.rangeEquals(RealBufferedSource.java:392)
        at okhttp3.internal.Util.bomAwareCharset(Util.java:431)
        at okhttp3.ResponseBody.string(ResponseBody.java:174)
        at com.example.appdevelopment_assignment_7.ComplexCurrencyConversionActivity$update$1.onResponse(ComplexCurrencyConversionActivity.kt:36)
        at okhttp3.RealCall$AsyncCall.execute(RealCall.java:141)
        at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:32)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1137)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:637)
        at java.lang.Thread.run(Thread.java:1012)

The problematic line is osw.write(response.body()?.string()). I hope someone might have an idea what could be the problem because I'm stuck here quite a while and couldn't figure it out.

Thanks in advance and Kind regards

You can call string() on ResponseBody only once, as okhttp reads the data from stream and does not hold reference, it throws closed as it has already closed the stream post reading! You need to call string() and store value inside a variable and then use it multiple times.

val request = Request.Builder().url(url).header("apikey", key).build()

val client = OkHttpClient()
client.newCall(request).enqueue(object : Callback {
    override fun onFailure(call: Call, e: IOException) {}
    override fun onResponse(call: Call, response: Response) {
        val responseBody = response.body()?.string() // stream closed after invoking string()
        println(responseBody)
        val date = LocalDate.now()
        val osw = OutputStreamWriter(openFileOutput("$date.json", MODE_PRIVATE))
        osw.write(responseBody)
        println("finished")
    }
})
}