为何Button点击没反应呢？：报错

XML文件中中代码：

<?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  xmlns:tools="http://schemas.android.com/tools"  android:id="@+id/activity_main"  android:layout_width="match_parent"  android:layout_height="match_parent"  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="com.ganhui.rtest4.MainActivity">


    <Button  android:text="Button"  android:layout_width="wrap_content"  android:layout_height="wrap_content"  android:layout_alignParentTop="true"  android:layout_centerHorizontal="true"  android:id="@+id/anniu"  android:clickable="true"/>
</RelativeLayout>

package com.ganhui.rtest4; import android.app.Activity; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.Toast; public class MainActivity extends Activity { private Button button; @Override  protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main); button= (Button) findViewById(R.id.anniu); button.setOnClickListener(new View.OnClickListener() { @Override  public void onClick(View v) {
                Toast.makeText(MainActivity.this,"123",Toast.LENGTH_SHORT);
            }
        });
    }
}