当我尝试运行我的应用程序时，我得到一个错误，上面写着“尝试在空对象引用上调用虚拟方法”，这是因为我试图通过主活动访问层列表中的项，并且我可能错误地引用了它们，这导致了我出现在这里。
我将layer-list.xml中的各个项设置为变量，它们是通过它们的id找到的。我的项目变量都返回为null。下面是我的代码：

import android.os.Bundle
import android.util.Log
import android.widget.Button
import android.widget.ImageView

import androidx.appcompat.app.AppCompatActivity
import androidx.constraintlayout.helper.widget.Layer

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

Log.d("EA","My array list of drawables is created next")
        val hairList = arrayListOf(R.drawable.hairponytailblue , R.drawable.hairponytailpink)
        val mouthList = arrayListOf(R.drawable.smilebigsmile, R.drawable.smileopenmouthfrown)
        val backgroundImageList = arrayListOf(R.drawable.background , R.drawable.background2)
Log.d("XX","done")

Log.d("EA","created the list iterators")
        var hairListIterator = hairList.iterator()
        var mouthListIterator = mouthList.iterator()
        var backgroundImageListIterator = backgroundImageList.iterator()

        Log.d("XX","done")

Log.d("EA","Below I created the variable for the images and buttons in use here, I also set my main images image resource to my layer-list.xml")

      //everything seems "fine" at this declaration but when I hover over R.drawable.layer , it returns its exact path on my computer in bright red, below that it says "@drawable/layer =>layer.xml"
      //
       val basePicture = findViewById<ImageView>(R.id.mainimage).setImageResource(R.drawable.layer)

        val hairButton = findViewById<Button>(R.id.hairbutton)
        val mouthButton = findViewById<Button>(R.id.mouthbutton)
        val backgroundButton = findViewById<Button>(R.id.backgroundbutton)

Log.d("XX","done")

 Log.d("EA","Here I set up the items/layers in layer.xml that I want to change")
        //these keep returning as null in debug but program continues on , I think its because of HOW they were referenced, these are items INSIDE a layer-list.xml.

        val hairLayer = findViewById<Layer>(R.id.layerHair)
        val mouthLayer = findViewById<Layer>(R.id.layerMouth)
        val backgroundLayer = findViewById<Layer>(R.id.layerBackground)

 Log.d("XX","Done")

 Log.d("EA", "Here I set the first of my arrays as each layers first images")

       /*error starts here,
       my error says : "Attempt to invoke virtual method 'void androidx.constraintlayout.helper.widget.Layer.setBackgroundResource(int)' on a null object reference "
       this has something to do with hairLayer...*/

        hairLayer.setBackgroundResource(hairList.first())
        mouthLayer.setBackgroundResource(mouthList.first())
        backgroundLayer.setBackgroundResource(backgroundImageList.first())

Log.d("XX","Done")

Log.d("EA","Here I told my list iterators to do something")

        hairButton.setOnClickListener {
            if( hairListIterator.hasNext()){
                hairLayer.setBackgroundResource(hairListIterator.next())

            }
            else{
                hairLayer.setBackgroundResource(hairList.first())
                hairListIterator = hairList.iterator()
            }
        }
Log.d("EA2","hairLayers new background resource is : " + hairLayer.drawableState)

Log.d("XX","done")

    }
}

我的目标与这些层列表项目，是改变他们的抽屉。我找到了一些方法，比如“mutate（）”和“setdrawablebylayerid（）”，但它们都与layerdrawable有关。根据androidstudio开发者网站，一个层列表编译一个layerdrawable？我很困惑。
p、 如果我的图层列表还不是一个layerdrawable，我该如何将它转换成一个呢？我觉得如果我可以在代码开头的某个地方将我的层列表转换为layerdrawable，那么我就不必不断地尝试引用layer-list.xml中的单个项。