Android出错The application has stopped unexpectedly

Android出错The application has stopped unexpectedly
刚开始学Android,编了一个简单的程序,代码如下就出现上面的错误了.是什么原因啊
public class Hello extends Activity {

private Button myButton = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
myButton = (Button)findViewById(R.id.myButton);
myButton.setText(R.string.welcome);
}
}

我暂时保留我的看法！