context.startActivity() does not start activity

Question

I am currently working on an alarm clock, which should start an activity after a certain time has passed. For this, I am using a PendingIntent with a BroadcastReceiver like shown below:

public class MainActivity extends AppCompatActivity {

    private AlarmManager alarmManager;
    private PendingIntent pendingIntent;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    public void btnAlarmSetWakeup(View view) {
        alarmManager = (AlarmManager) this.getSystemService(Context.ALARM_SERVICE);
        Intent intent = new Intent(this, AlarmReceiver.class);
        pendingIntent = PendingIntent.getBroadcast(this, 0, intent, 0);
        alarmManager.set(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + 3000, pendingIntent);
    }

public class AlarmReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        Intent alarmIntent = new Intent(context, WakeupAlarmActivity.class);
        alarmIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

        context.startActivity(alarmIntent);
    }
}

public class WakeupAlarmActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_wakeup_alarm);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1) {

            setShowWhenLocked(true);
            setTurnScreenOn(true);
            KeyguardManager keyguardManager = (KeyguardManager) getSystemService(this.KEYGUARD_SERVICE);
            keyguardManager.requestDismissKeyguard(this, null);
        }
        else {

            getWindow().addFlags(WindowManager.LayoutParams.FLAG_DISMISS_KEYGUARD |
                    WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED |
                    WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON |
                    WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        }
    }

}

Unfortunately, after 3 seconds the AlarmReceiver class gets executed, but it doesn't start the WakeupAlarmReceiver activity. Am I doing something wrong?