Activity aus einem BroadcastReceiver "OnReceive" heraus starten

<receiver android:name=".SMSReceiver"
            android:enabled="true"
            android:exported="true"
            android:permission="android.permission.BROADCAST_SMS">
            <intent-filter android:priority="2147483647" >
                <action android:name="android.provider.Telephony.SMS_RECEIVED" />
            </intent-filter>
        </receiver>
        <activity android:name=".SecondActivity" android:launchMode="singleTop" />
        <activity android:name=".MainActivity" android:launchMode="singleTop">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

public void onReceive(Context context, Intent intent) {
    ... Auswertung der SMS ...
    // Start der zweiten Activity
    Intent showDialog = new Intent();
    showDialog.setClassName(SecondActivity.class.getPackage().getName(),SecondActivity.class.getName());
    showDialog.setAction(Intent.ACTION_VIEW);
    showDialog.addCategory(Intent.CATEGORY_LAUNCHER);
    showDialog.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY);
    context.startActivity(showDialog);
    ...
}

public class SMSReceiver extends BroadcastReceiver {


    /**
     * Event bei Empfang einer SMS den Service starten
     * @param context
     * @param intent
     */
    @Override
    public void onReceive(Context context, Intent intent) {
        Intent iStartServive = new Intent(context, BellMyPhoneService.class);
        iStartServive.setAction(BellMyPhoneApp.ACTION_START_SMSCASTSERVICE);
        // die SMS in den StarteService-Intent legen
        iStartServive.putExtras(intent.getExtras());
        context.startService(iStartServive);
    }

/**
     * Your Have Find Me - Dialog anzeigen
     */
    private void showNotification() {
        String chanelId = createNotificationChanel();


        // Event für SoundPlayerStop
        Intent iStopSound = new Intent();
        iStopSound.setAction(BellMyPhoneApp.ACTION_FOR_STOP_SOUND_PLAYER);
        PendingIntent piStopSound = PendingIntent.getBroadcast(this, 0, iStopSound, 0);


        // Notification erzeugen
        Intent iFullScreen = new Intent(this, BellMyPhoneService.class);
        iFullScreen.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_SINGLE_TOP);
        PendingIntent piFullScreen = PendingIntent.getActivity(this, 0, iFullScreen, PendingIntent.FLAG_UPDATE_CURRENT);
        NotificationCompat.Builder nbShowDialog = new NotificationCompat.Builder(this, chanelId)
                .setDefaults(NotificationCompat.DEFAULT_ALL)
                .setNumber(BellMyPhoneApp.NOTIFICATION_ID)
                .setSmallIcon(R.drawable.ic_launcher_foreground)
                .setContentTitle(getResources().getString(R.string.strvalue_app_name))
                .setContentIntent(piFullScreen)
                .setContentText(getResources().getString(R.string.strvalue_foundme_txt))
                .addAction(0, getResources().getString(R.string.strvalue_okay_txt),piStopSound)
                .setPriority(NotificationCompat.PRIORITY_HIGH)
                //.setFullScreenIntent(piFullScreen, true)
                .setAutoCancel(true);
        if (bellSeconds > 0) {
            nbShowDialog.setProgress(bellSeconds,0,false);
        }
        Notification nShowDialog = nbShowDialog.build();
        startForeground(BellMyPhoneApp.NOTIFICATION_ID, nShowDialog);
        stopForeground(false);
    }


    /**
     * erzeugt den NotificationChannel
     */
    private String createNotificationChanel(){
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
        {
            NotificationChannel notiChan = new NotificationChannel(
                    BellMyPhoneApp.CHANNEL_ID,  BellMyPhoneApp.APP_NAME, NotificationManager.IMPORTANCE_HIGHT
            );
            notiChan.setLockscreenVisibility(Notification.VISIBILITY_PUBLIC);
            notiChan.enableLights(true);
            notiChan.setLightColor(Color.YELLOW);
            notiChan.setDescription(getResources().getString(R.string.strvalue_chanel_descript));
            NotificationManager nManager = getSystemService(NotificationManager.class);
            nManager.createNotificationChannel((notiChan));
            return BellMyPhoneApp.CHANNEL_ID;
        }
        return "";
    }

/**
     * Aufruf der Servcice-Aktion
     *
     * @param intent
     * @param flags
     * @param startId
     * @return
     */
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        if ((intent != null) && (intent.getAction() != null)) {
            if (intent.getAction().equals(BellMyPhoneApp.ACTION_START_SMSCASTSERVICE)) {
                checkSMS(intent);
            }
        }
        return START_NOT_STICKY;
    }