Android Studio编译无错，但在模拟器上无法运行App

今天在学习郭霖老师的《第一行代码》第三版时碰到一个问题，明明运行无错，却无法在模拟器上正常运行。一开始提示说“Waiting for all target devices to come online”，在百度后进行了Cold Boot Now 和 Wipe Data也无用。大佬们有没有懂的帮忙看看，阿里嘎多。

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

fruit_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="5dp">

    <ImageView
        android:id="@+id/fruitImage"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="10dp"/>
    <TextView
        android:id="@+id/fruitName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="left"
        android:layout_marginTop="10dp" />

</LinearLayout>

MainActivity.kt

package com.example.listview

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ImageView
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.RecyclerView
import androidx.recyclerview.widget.StaggeredGridLayoutManager
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    private val fruitList = ArrayList<Fruit>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initFruits() // 初始化水果数据
        val layoutManager = StaggeredGridLayoutManager(3, StaggeredGridLayoutManager.VERTICAL)
        recyclerView.layoutManager = layoutManager
        val adapter = FruitAdapter(fruitList)
        recyclerView.adapter = adapter

    }

    class Fruit(val name:String, val imageId: Int)

    class FruitAdapter(val fruitList: List<Fruit>) : RecyclerView.Adapter<FruitAdapter.ViewHolder>() {

        inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
            val fruitImage: ImageView = view.findViewById(R.id.fruitImage)
            val fruitName: TextView = view.findViewById(R.id.fruitName)
        }

        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
            val view = LayoutInflater.from(parent.context).inflate(R.layout.fruit_item, parent, false)
            return ViewHolder(view)
        }

        override fun onBindViewHolder(holder: ViewHolder, position: Int) {
            val fruit = fruitList[position]
            holder.fruitImage.setImageResource(fruit.imageId)
            holder.fruitName.text = fruit.name
        }

        override fun getItemCount() = fruitList.size

    }

    private fun initFruits() {
        repeat(2) {
            fruitList.add(Fruit(getRandomLengthString("Apple"),
                R.drawable.apple_pic))
            fruitList.add(Fruit(getRandomLengthString("Banana"),
                R.drawable.banana_pic))
            fruitList.add(Fruit(getRandomLengthString("Orange"),
                R.drawable.orange_pic))
            fruitList.add(Fruit(getRandomLengthString("Watermelon"),
                R.drawable.watermelon_pic))
            fruitList.add(Fruit(getRandomLengthString("Pear"),
                R.drawable.pear_pic))
            fruitList.add(Fruit(getRandomLengthString("Grape"),
                R.drawable.grape_pic))
            fruitList.add(Fruit(getRandomLengthString("Pineapple"),
                R.drawable.pineapple_pic))
            fruitList.add(Fruit(getRandomLengthString("Strawberry"),
                R.drawable.strawberry_pic))
            fruitList.add(Fruit(getRandomLengthString("Cherry"),
                R.drawable.cherry_pic))
            fruitList.add(Fruit(getRandomLengthString("Mango"),
                R.drawable.mango_pic))
        }
    }

    private fun getRandomLengthString(str: String): String {
        val n = (1..20).random()
        val builder = StringBuilder()
        repeat(n) {
            builder.append(str)
        }
        return builder.toString()
    }

}