java.lang.RuntimeException：使用Kotlin执行doInBackground（）时发生错误

override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_main)
            callPostReq()
            val sm = someTask()
            sm.execute()
        }
inner class someTask(): AsyncTask<String, Int, Array<String>>() {
    override fun doInBackground(vararg params: String): Array<String>?  {
        val conn: Connection= null!!
        val url = "jdbc:mysql://localhost:3306/"
        val dbName = "db-01"
        val driver = "com.mysql.jdbc.Driver"
        val cred = Properties()
        cred.put("username","username")
        cred.put("password","pasword")

        try {
            Class.forName(driver).newInstance()
            conn = DriverManager.getConnection(url,cred)
            println("Connected to the database")
            conn.close()
            println("Disconnected from database")
        } catch (e: Exception) {
            e.printStackTrace()
        }
        return null
    }
}

致命异常：AsyncTask＃1进程：highway.dueamount，PID：15746 java.lang.RuntimeException：在java.util上的android.os.AsyncTask $ 3.done（AsyncTask.java:325）执行doInBackground（）时发生错误.concurrent.FutureTask.finishCompletion（FutureTask.java:354）at java.util.concurrent.FutureTask.setException（FutureTask.java:223）at java.util.concurrent.FutureTask.run（FutureTask.java:242）at android . os.AsyncTask $ SerialExecutor $ 1.run（AsyncTask.java:243）at java.util.concurrent.ThreadPoolExecutor.runWorker（ThreadPoolExecutor.java:1133）at java.util.concurrent.ThreadPoolExecutor $ Worker.run（ThreadPoolExecutor.java:607） ）在java.lang.Thread.run（Thread.java:760）引起：ktlin.KotlinNullPointerException at highway.dueamount.MainActivity $ someTask.doInBackground（MainActivity.kt：60）at highway.dueamount.MainActivity $ someTask.doInBackground（ MainActivity.kt：57）在android.os.AsyncTask $ 2.call（AsyncTask.java:305）在android.os的java.util.concurrent.FutureTask.run（FutureTask.java:237） .AsyncTask $ SerialExecutor $ 1.run（AsyncTask.java:243）java.util.concurrent.ThreadPoolExecutor.runWorker（ThreadPoolExecutor.java:1133）at java.util.concurrent.ThreadPoolExecutor $ Worker.run（ThreadPoolExecutor.java:607）在java.lang.Thread.run（Thread.java:760）