java.lang.IllegalStateException You need to use a Theme.AppCompat theme (or descendant) with this activity

Greetings my developer friends!

I created a demo app to test the material design for minSdk 14 and targetSdj 23. I have the following files below. It works fine below the 21 API. I believe the culprit is Styles-v21.xml as the app worked fine before that.

Its a simple setup, just one activity. I am testing the Material theme and AppCompat theme for working with the following scenario.

1. Theme.AppCompat for API below 21.
2. Theme.Material for API 21 and 21+.

The app works fine below API 21, but above 21+ the App crashes saying,"Unfortunately materialTest has stopped."

The error is as follows:

FATAL EXCEPTION: main
        Process: net.slidenerd.materialtest, PID: 28204
        java.lang.RuntimeException: Unable to start activity ComponentInfo{net.slidenerd.materialtest/net.slidenerd.materialtest.MainActivity}: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
        at android.app.ActivityThread.-wrap11(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
        Caused by: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
        at android.support.v7.app.AppCompatDelegateImplV7.createSubDecor(AppCompatDelegateImplV7.java:340)
        at android.support.v7.app.AppCompatDelegateImplV7.ensureSubDecor(AppCompatDelegateImplV7.java:309)
        at android.support.v7.app.AppCompatDelegateImplV7.setContentView(AppCompatDelegateImplV7.java:273)
        at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:139)
        at net.slidenerd.materialtest.MainActivity.onCreate(MainActivity.java:12)
        at android.app.Activity.performCreate(Activity.java:6237)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
        at android.app.ActivityThread.-wrap11(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

MainActivity.java

package net.slidenerd.materialtest;

import android.app.Activity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

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

Styles.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>

    </style>

</resources>

Styles-v21.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="AppTheme" parent="android:Theme.Material.Light.NoActionBar">
        <item name="android:colorPrimary">@color/colorPrimary</item>
        <item name="android:colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="android:colorAccent">@color/colorAccent</item>

    </style>

</resources>

Manifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="net.slidenerd.materialtest">

    <application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">

        <activity android:name=".MainActivity"
        android:theme="@style/AppTheme"
        >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
               <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Solution: I solved this issue by extending Activity instead of AppCompatActivity and the application runs fine on both API 14+ to API 23 smoothly. But, could you justify the preferred solution to this problem.

All the code needed to produce the error is above.

Note: To produce the error, MainActivity should extend AppCompatActivity and you should run on API 21 or above.

Could anyone explain the ideal case happening here so as to understand the underlying problem?

Thank You!