kotlin 中的“NullPointerException：null 无法转换为非 null 类型错误”

问：

 var notify = ArrayList<String>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main_user)
        btn_list2.setOnClickListener {

            val sharedPreferences1 = getSharedPreferences("id", Context.MODE_PRIVATE)
            val documentid: String? = sharedPreferences1.getString("id","null")
            val c = FirebaseFirestore.getInstance()
            val d = c.collection("applicationForm").document(documentid.toString()).get()
                .addOnSuccessListener { document ->

                    notify = document?.get("notifyTo") as ArrayList<String>

                    var str1 = notify.joinToString()
                    Toast.makeText(applicationContext,str1,Toast.LENGTH_SHORT).show()

}}}

在这里，我在行中出现错误。notify = document?.get("notifyTo") as ArrayList<String>

这是我的logcat详细信息

java.lang.NullPointerException: null cannot be cast to non-null type java.util.ArrayList<kotlin.String>
        at com.example.bloodbankcompany.MainActivityUser.onCreate$lambda-4$lambda-3(MainActivityUser.kt:47)
        at com.example.bloodbankcompany.MainActivityUser.lambda$cGlrfLFSOO25IeEAacXMuz6Tzx0(Unknown Source:0)`. 

请谁能帮忙。在这里，我正在尝试从firestore读取数组文档。