in chapter 3 on the book under topic Applying themes to an activity.
When i tried to add android:theme="@android:style/Theme.Material"
showed 2 warning. After ignoring warning i ran the app then it kept on crashing.
warning 1:App is not indexable by Google Search; consider adding at least one Activity with an ACTION-VIEW intent filter.
warning 2:On SDK version 23 and up, your app data will be automatically backed up and restored on app install. Consider adding the attribute android:fullBackupContent to specify an @xml resource which configures which files to backup.

I am having the same issue, have you resolved it? Because I am not able to build anything after chapter 2

I had no problem with the code in Chapter 3. Make sure you have the closing ">" as below.
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@android:style/Theme.Material">
<activity
android:name=".MainActivity"
android:label="@string/app_name"
android:theme="@style/AppTheme.NoActionBar">
<intent-filter>
<action android:name="android.intent.action.MAIN" />

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