Android java.lang.NullPointerException:尝试调用虚拟方法。。关于空对象引用

我正在尝试编写一个联系人程序，用户按下按钮添加联系人（在第二个活动中）

我得到一个错误：

java.lang.NullPointerException:尝试对空对象引用调用虚拟方法“android.view.Window$Callback android.view.Window.getCallback（）”

以下是我的主要活动：

public class MainActivity extends AppCompatActivity {

private ListView lvContact;
private Button bt;
@Override

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

    final List<ContactBook> listContactBook = new ArrayList<ContactBook>();
    lvContact = (ListView)findViewById(R.id.listcontact);
    final ContactBookAdapter adapter = new ContactBookAdapter(this, listContactBook);
    lvContact.setAdapter(adapter);

    lvContact.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
            Intent intent = new Intent(getApplicationContext(), secondActivity.class);
            startActivity(intent);

        }
    });
    try {
        bt = (Button) findViewById(R.id.add);

        bt.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(getApplicationContext(), secondActivity.class);
                startActivity(intent);
            }

        });
    }
    catch(NullPointerException e){
        System.out.checkError();
    }

---

列表视图项未正确使用单击。您需要在列表视图布局中添加此行：

android:descendantFocusability="blocksDescendants"    

---

在下面的线上放一个断点

lvContact = (ListView)findViewById(R.id.listcontact);

---

然后，尝试调试应用程序，从堆栈跟踪开始，listview对象为null，再次勾选此R.id.listcontact，或者在activity_second.xml中更改listview的id，然后在第二个activity中更新它。这可能会对您有所帮助。

显示完整错误日志的可能副本。问题已用完整错误日志编辑！调试代码以查找代码中的空对象。适配器为空

I/PhoneWindow: [setNavigationBarColor2] color=0x ff000000
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.bignerdranch.android.contactapp, PID: 16741
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bignerdranch.android.contactapp/com.bignerdranch.android.contactapp.secondActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2378)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2440)
                  at android.app.ActivityThread.access$800(ActivityThread.java:162)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1348)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:135)
                  at android.app.ActivityThread.main(ActivityThread.java:5422)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:372)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:914)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:707)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
                  at com.bignerdranch.android.contactapp.secondActivity.onCreate(secondActivity.java:37)
                  at android.app.Activity.performCreate(Activity.java:6057)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2331)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2440) 
                  at android.app.ActivityThread.access$800(ActivityThread.java:162) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1348) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:135) 
                  at android.app.ActivityThread.main(ActivityThread.java:5422) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at java.lang.reflect.Method.invoke(Method.java:372) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:914) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:707) 

android:descendantFocusability="blocksDescendants"    

lvContact = (ListView)findViewById(R.id.listcontact);