安卓应用开发在点击按钮时闪退

Question

已经解决了，原来是没给按钮加tag
(¦3[▓▓]记录一下吧
编写计算器程序的时候出现了闪退，所以没有写完，只有一部分功能，大概是：
希望在点击数字按钮的时候，第一行TextView能够显示点击的数字，并在点击第二个数字时与前一个数字进行拼接显示。
虽然是照着别人做的，但是由于kotlin-android-extensions插件被废止了，所以我做的时候就不能直接引用xml中的id，只好现学用ViewBinding来代替findViewById的工作。
虽然程序没报错，却在点击按钮时出现了闪退，求帮助，以下是我的MainActivity的代码。

class MainActivity : ComponentActivity() {
    private lateinit var binding:ActivityMainBinding
    var num1: StringBuilder = StringBuilder() 
    var num2: StringBuilder = StringBuilder()
    lateinit var operation: String //还没用
    var isFirstNumber: Boolean = true
    var currentOperation: String? = null //还没用
    var details: StringBuilder = StringBuilder()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)
        binding.oneBtn.setOnClickListener {
            numClicked(it)
        }
        binding.twoBtn.setOnClickListener {
            numClicked(it)
        }
        binding.threeBtn.setOnClickListener {
            numClicked(it)
        }
        binding.fourBtn.setOnClickListener {
            numClicked(it)
        }
        binding.fiveBtn.setOnClickListener {
            numClicked(it)
        }
        binding.sixBtn.setOnClickListener {
            numClicked(it)
        }
        binding.eightBtn.setOnClickListener {
            numClicked(it)
        }
        binding.nineBtn.setOnClickListener {
            numClicked(it)
        }
        binding.zeroBtn.setOnClickListener {
            numClicked(it)
        }

    }

    fun numClicked(view: View){
        var numString = view.tag as String
        if(isFirstNumber){
            num1.append(numString)  //把数加在操作符前的数后
            details.append(numString)
        }
        else{
            num2.append(numString)
            details.append(numString)
        }
        binding.progressTextView.text = "$details" //展示字符串
    }

还有日志，理解不了T_T

20:53:16.168  E  FATAL EXCEPTION: main
                 Process: com.example.mycalculator, PID: 8819
                 java.lang.NullPointerException: null cannot be cast to non-null type kotlin.String
                     at com.example.mycalculator.MainActivity.numClicked(MainActivity.kt:62)
                     at com.example.mycalculator.MainActivity.onCreate$lambda$4(MainActivity.kt:44)
                     at com.example.mycalculator.MainActivity.$r8$lambda$UgAc9_AiizbUhm38KbDJtqxZQWI(Unknown Source:0)
                     at com.example.mycalculator.MainActivity$$ExternalSyntheticLambda4.onClick(Unknown Source:2)
                     at android.view.View.performClick(View.java:7125)
                     at android.view.View.performClickInternal(View.java:7102)
                     at android.view.View.access$3500(View.java:801)
                     at android.view.View$PerformClick.run(View.java:27336)
                     at android.os.Handler.handleCallback(Handler.java:883)
                     at android.os.Handler.dispatchMessage(Handler.java:100)
                     at android.os.Looper.loop(Looper.java:214)
                     at android.app.ActivityThread.main(ActivityThread.java:7356)
                     at java.lang.reflect.Method.invoke(Native Method)
                     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
20:53:16.192  I  Sending signal. PID: 8819 SIG: 9
20:53:16.291  E  channel '7145893 com.example.mycalculator/com.example.mycalculator.MainActivity (server)' ~ Channel is unrecoverably broken and will be disposed!