Android容器-工具栏上的SearchView [帮助]

Question

我遇到了一个奇怪的问题，我的应用程序正在停止，但是在Android Studio的构建中没有出现错误。

我有MainActivity.java，activity_main.xml（线性布局），search_menu.xml（在res / menu内部）

MainActivity.java

package com.readabook.readabookonline;

import android.support.v4.view.MenuItemCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.SearchView;
import android.widget.Toast;


import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    ListView searchBtn;
    ArrayList<String> listItem;
    ArrayAdapter<String> listAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        searchBtn = findViewById(R.id.listViewdemo);

        listItem.add("item1");
        listItem.add("item2");
        listItem.add("item3");
        listItem.add("item4");
        listItem.add("item5");
        listItem.add("item6");

        listAdapter = new ArrayAdapter<>(this,android.R.layout.activity_list_item, listItem);
        searchBtn.setAdapter(listAdapter);


    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu){
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.search_menu, menu);
        MenuItem searchViewItem = menu.findItem(R.id.bar_search);
        final SearchView searchView = (SearchView) MenuItemCompat.getActionView(searchViewItem);
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String query) {
                searchView.clearFocus();
                if (listItem.contains(query)) {
                    listAdapter.getFilter().filter(query);
                } else {
                    Toast.makeText(getApplicationContext(), "No match found", Toast.LENGTH_LONG).show();
                }
                return false;
            }

            @Override
            public boolean onQueryTextChange(String newText) {
                listAdapter.getFilter().filter(newText);
                return false;
            }
        });
        return super.onCreateOptionsMenu(menu);
    }
}

activity_main.xml

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

    <ListView
        android:id="@+id/listViewdemo"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_margin="10dp" />

</LinearLayout>

search_menu.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <item
        android:id="@+id/bar_search"
        android:icon="@mipmap/ic_launcher_round"
        android:title="Search"
        app:actionViewClass="android.widget.SearchView"
        app:showAsAction="ifRoom|withText" />
</menu>

Logcat

2019-04-20 13:15:33.949 15594-15594/com.readabook.readabookonline E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.readabook.readabookonline, PID: 15594
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.readabook.readabookonline/com.readabook.readabookonline.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
        at com.readabook.readabookonline.MainActivity.onCreate(MainActivity.java:30)
        at android.app.Activity.performCreate(Activity.java:7009)
        at android.app.Activity.performCreate(Activity.java:7000)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

我希望在应用程序的右上角显示搜索图标，但突然该应用程序崩溃了。