"Default Activity Not Found" on Android Studio upgrade

If you see that error occur after upgrading versions of IntelliJ IDEA or Android Studio, or after generating a new APK file, you may need to refresh the IDE's cache.

Menu File → Invalidate Caches and restart...

I can't comment on why the upgrade of IntelliJ IDEA might cause this problem because I don't use it.

However, that error: "Default Activity Not Found" seems to be telling you that you don't have an activity declared in file AndroidManifest.xml that is marked as the main activity, to be launched when the application starts.

You should have at least one activity that looks something like this:

<activity
        android:name="com.your.package.name.YourActivity"
        android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

If you don't have at least one activity with an intent filter like that, you would most likely see the error message you have included here.

You should add that intent filter to the Activity that you wish to open when you start the application, and that should fix your problem.

Additional details

(Android Studio 4.1.2) if the project is created as EmptyApplication then the developer must manually create below three files to avoid the Default Activity Not Found error:

File AndroidManifest.xml

File MainActivity.java

File activity_main.xml