写了个Android电话拨号器，运行时提示错误：Caused by:java.lang.NullpointerException 代码如下：

package com.test.dail;

import android.app.Activity;
import android.app.Fragment;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;public class MainActivity extends Activity

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

              Button bt_dail = (Button)this.findViewById(R.id.bt_dail);

              bt_dail.setOnClickListener(new MyListener());

              if (savedInstanceState == null)
              {
                    getFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
             }
}

public class MyListener implements OnClickListener
{
      //当按钮被点击的时候调用的方法
      @Override
      public void onClick(View v)
     {
        EditText et_nuber = (EditText)MainActivity.this.findViewById(R.id.et_number);

        String number =et_nuber.getText().toString();

        Intent intent = new Intent(); //创建一个意图

        intent.setAction(Intent.ACTION_CALL);

        intent.setData(Uri.parse("tel:" + number));

       startActivity(intent);

     }

}

  @Override
public boolean onCreateOptionsMenu(Menu menu)
      // Inflate the menu; this adds items to the action bar if it is present.

      getMenuInflater().inflate(R.menu.main, menu);
      return true;
@Override

public boolean onOptionsItemSelected(MenuItem item)
{
      // Handle action bar item clicks here. The action bar will
      // automatically handle clicks on the Home/Up button, so long
     // as you specify a parent activity in AndroidManifest.xml.
     int id = item.getItemId();
     if (id == R.id.action_settings)
    {
       return true;
    }
       return super.onOptionsItemSelected(item);
}

/**
* A placeholder fragment containing a simple view.
*/
    public static class PlaceholderFragment extends Fragment
   {

           public PlaceholderFragment()
        {
        } 

         @Override

          public View onCreateView(LayoutInflater inflater, ViewGroup                 container,Bundle savedInstanceState)

        {

           View rootView = inflater.inflate(R.layout.fragment_main, container,
 false);
          return rootView;
       }
   }

}