java.lang.NullPointerException at com.example.phone.MainActivity.onCreate

写个android拨号器，运行是报如下错误

04-09 01:36:33.880: E/AndroidRuntime(3689): Caused by: java.lang.NullPointerException
04-09 01:36:33.880: E/AndroidRuntime(3689): at com.example.phone.MainActivity.onCreate(MainActivity.java:30)

网上找了很久也没有找到答案，请高手指点下

代码如下：

1 package com.example.phone;

2
3 import android.support.v7.app.ActionBarActivity;
4 import android.support.v7.app.ActionBar;
5 import android.support.v4.app.Fragment;
6 import android.text.TextUtils;
7 import android.content.Intent;
8 import android.net.Uri;
9 import android.os.Bundle;
10 import android.view.LayoutInflater;
11 import android.view.Menu;
12 import android.view.MenuItem;
13 import android.view.View;
14 import android.view.ViewGroup;
15 import android.view.View.OnClickListener;
16 import android.widget.Button;
17 import android.widget.EditText;
18 import android.widget.Toast;
19 import android.os.Build;
20
21 public class MainActivity extends ActionBarActivity {
22         public EditText call_number;
23
24         @Override
25         protected void onCreate(Bundle savedInstanceState) {
26                 super.onCreate(savedInstanceState);
27                 setContentView(R.layout.activity_main);
28                 Button bt_call = (Button) findViewById(R.id.button);
29                 call_number = (EditText) this.findViewById(R.id.et_number);
30                 bt_call.setOnClickListener(new mybuttonclick());
31                 if (savedInstanceState == null) {
32                         getSupportFragmentManager().beginTransaction()
33                                         .add(R.id.container, new PlaceholderFragment()).commit();
34                 }
35         }
36
37         private class mybuttonclick implements OnClickListener {
38
39                 @Override

40                 public void onClick(View v) {
41                         String number = call_number.getText().toString().trim();
42                         if (TextUtils.isEmpty(number)) {
43                                 Toast.makeText(MainActivity.this, "号码不能为空", Toast.LENGTH_SHORT)
44                                                 .show();
45                                 return;
46                         }
47                         Intent intent = new Intent();
48                         intent.setAction(Intent.ACTION_CALL);
49                         intent.setData(Uri.parse("tel:" + number));
50                         startActivity(intent);
51                 }
52
53         }
54
55         /**
56          * A placeholder fragment containing a simple view.
57          */
58         public static class PlaceholderFragment extends Fragment {
59
60                 public PlaceholderFragment() {
61                 }
62
63                 @Override
64                 public View onCreateView(LayoutInflater inflater, ViewGroup container,
65                                 Bundle savedInstanceState) {
66                         View rootView = inflater.inflate(R.layout.fragment_main, container,
67                                         false);
68                         return rootView;
69                 }
70         }
71
72 }

运行时报如下错误，求助

04-09 01:36:33.880: W/dalvikvm(3689): threadid=1: thread exiting with uncaught exception (group=0x40bcc300)
04-09 01:36:33.880: E/AndroidRuntime(3689): FATAL EXCEPTION: main
04-09 01:36:33.880: E/AndroidRuntime(3689): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.phone/com.example.phone.MainActivity}: java.lang.NullPointerException
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.ActivityThread.access$700(ActivityThread.java:131)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1228)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.os.Handler.dispatchMessage(Handler.java:99)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.os.Looper.loop(Looper.java:137)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.ActivityThread.main(ActivityThread.java:4866)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at java.lang.reflect.Method.invokeNative(Native Method)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at java.lang.reflect.Method.invoke(Method.java:511)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at dalvik.system.NativeStart.main(Native Method)
04-09 01:36:33.880: E/AndroidRuntime(3689): Caused by: java.lang.NullPointerException
04-09 01:36:33.880: E/AndroidRuntime(3689):    at com.example.phone.MainActivity.onCreate(MainActivity.java:30)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.Activity.performCreate(Activity.java:5143)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
04-09 01:36:33.880: E/AndroidRuntime(3689):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074)
04-09 01:36:33.880: E/AndroidRuntime(3689):    ... 11 more

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