使用Kotlin时出现NumberFormatException错误

Question

我一直在使用Kotlin开发一个小费计算器应用程序，我遇到了一个我似乎无法修复的错误，我已经绕过互联网，我无法找到明确的答案。

这是我的代码：

package com.jackjessop.testapp

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import android.widget.TextView

class MainActivity : AppCompatActivity() {

 override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
     setContentView(R.layout.activity_main)

     val billAmountTV: TextView = findViewById(R.id.billAmountText)
     val tipPercentageTV: TextView = findViewById(R.id.tipPercentageText)

     val billAmount = billAmountTV.text.toString().toInt()
     val tipPercentage = tipPercentageTV.text.toString().toInt()

     val percentage = tipPercentage / billAmount
     val tipAmount =  billAmount * percentage
     val total = billAmount + tipAmount

     val tipText: TextView = findViewById(R.id.tipText)
     val calculateBtn: Button = findViewById(R.id.calculateButton)

     calculateBtn.setOnClickListener {
        tipText.text = total.toString()
     }
  } 
}

这是错误：

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.jackjessop.testapp, PID: 30234
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.jackjessop.testapp/com.jackjessop.testapp.MainActivity}: java.lang.NumberFormatException: For input string: ""
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2779)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2844)
                  at android.app.ActivityThread.-wrap12(ActivityThread.java)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1572)
                  at android.os.Handler.dispatchMessage(Handler.java:110)
                  at android.os.Looper.loop(Looper.java:203)
                  at android.app.ActivityThread.main(ActivityThread.java:6364)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)
               Caused by: java.lang.NumberFormatException: For input string: ""
                  at java.lang.Integer.parseInt(Integer.java:533)
                  at java.lang.Integer.parseInt(Integer.java:556)
                  at com.jackjessop.testapp.MainActivity.onCreate(MainActivity.kt:17)
                  at android.app.Activity.performCreate(Activity.java:6666)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2732)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2844) 
                  at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1572) 
                  at android.os.Handler.dispatchMessage(Handler.java:110) 
                  at android.os.Looper.loop(Looper.java:203) 
                  at android.app.ActivityThread.main(ActivityThread.java:6364) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)