Problems with page transition

I'm trying to create a simple page-changing event when I click the button, but every time I run the application it crashes when it opens.

MainActivity:

package com.app.agrandesacada;

import android.support.v4.app.Fragment;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.os.Build;

public class MainActivity extends Activity {
private Button btnAlunos;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    btnAlunos=(Button)findViewById(R.id.btnaluno);

    btnAlunos.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
        Intent i = new Intent("com.app.agrandesacada.AlunoActivity");
        startActivity(i);
        finish();

        }
    });

}


@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;
}


}

ManifestXML:

<?xml version="1.0" encoding="utf-8"?>

<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="17" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.app.agrandesacada.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name="com.app.agrandesacada.AlunoActivity"
        android:label="@string/title_activity_aluno" >
        <intent-filter>
            <action android:name="com.app.agrandesacada.AlunoActivity" />

            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity>
</application>

Error:

  

04-29 20: 45: 51,941: D / AndroidRuntime (511): Shutting down VM   04-29 20: 45: 51,941: W / dalvikvm (511): threadid = 1: thread exiting with uncaught exception (group = 0x40015560)   04-29 20: 45: 51,961: E / AndroidRuntime (511): FATAL EXCEPTION: main   04-29 20: 45: 51.961: E / AndroidRuntime (511): java.lang.RuntimeException: Unable to start activity ComponentInfo {com.app.agrandesacada / com.app.agrandesacada.MainActivity}: java.lang.NullPointerException   04-29 20: 45: 51.961: E / AndroidRuntime (511): at android.app.ActivityThread.performLaunchActivity (ActivityThread.java:1647)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at android.app.ActivityThread.handleLaunchActivity (ActivityThread.java:1663)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at android.app.ActivityThread.access $ 1500 (ActivityThread.java:117)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at android.app.ActivityThread $ H.handleMessage (ActivityThread.java:931)   04-29 20: 45: 51.961: E / AndroidRuntime (511): at android.os.Handler.dispatchMessage (Handler.java:99)   04-29 20: 45: 51.961: E / AndroidRuntime (511): at android.os.Looper.loop (Looper.java:123)   04-29 20: 45: 51.961: E / AndroidRuntime (511): at android.app.ActivityThread.main (ActivityThread.java:3683)   04-29 20: 45: 51.961: E / AndroidRuntime (511): at java.lang.reflect.Method.invokeNative (Native Method)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at java.lang.reflect.Method.invoke (Method.java:507)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run (ZygoteInit.java:839)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:597)   04-29 20: 45: 51.961: E / AndroidRuntime (511): at dalvik.system.NativeStart.main (Native Method)   04-29 20: 45: 51,961: E / AndroidRuntime (511): Caused by: java.lang.NullPointerException   04-29 20: 45: 51,961: E / AndroidRuntime (511): at com.app.agrandesacada.MainActivity.onCreate (MainActivity.java:24)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at android.app.Instrumentation.callActivityOnCreate (Instrumentation.java:1047)   04-29 20: 45: 51,961: E / AndroidRuntime (511): at android.app.ActivityThread.performLaunchActivity (ActivityThread.java:1611)   04-29 20: 45: 51.961: E / AndroidRuntime (511): ... 11 more

It gives NullPointException but I do not understand why.