This error will cause when your app lose define of your main activity, mostly is because you removed part of your main activity code from AndroidManifest.xml file.

Here is a sample of how your main activity class should be defined in AndroidManifest.xml file.

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

If you cross check your AndroidManifest.xml  with sample code above most probably you will realize that there is shorter code in  <intent-filter> section both action and category tags are required to define main activity.