android开发中Caused by: java.lang.NullPointerException

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >
    <RatingBar
        android:id="@+id/mybar1"
        android:numStars="4"
        android:stepSize="0.5"
        android:isIndicator="false"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
     <RatingBar
        android:id="@+id/mybar2"
        android:numStars="4"
        android:isIndicator="true"
        android:rating="2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:id="@+id/mytext"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</LinearLayout>

mainActivity.java

package com.example.ratingbarproject1;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.RatingBar;
import android.widget.RatingBar.OnRatingBarChangeListener;
import android.widget.TextView;
public class MainActivity extends Activity {
	private RatingBar ratingbar=null;
	private TextView textview=null;
	protected void onCreate(Bundle savedInstanceState) {
		this.ratingbar=(RatingBar)super.findViewById(R.id.mybar1);//获取组件
		this.textview=(TextView)super.findViewById(R.id.mytext);
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		this.ratingbar.setOnRatingBarChangeListener(new OnRatingBarChangeListener1());//设置监听
	}
private class OnRatingBarChangeListener1 implements OnRatingBarChangeListener{
	//@Override
	public void onRatingChanged(RatingBar ratingBar, float rating,
	boolean fromUser) {
		// 在显示文本中显示星星
		MainActivity.this.textview.append("***当前的星评分："
		+ratingBar.getNumStars()+"***当前的步长："+ratingBar.getStepSize()+"\n");

	}
}
	@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;
	}
}

错误。