Error getting strings with AsyncTask Android

I'm developing an application using DynamoDB, in a few words: i'm getting information from the database and I want to display this information.

I don't know what I supposed to do but i have an error inside the AsyncTask, the app is crushing always when i want to save this strings in an ArrayList. Here is my code:

private class db extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... params) {

        AmazonDynamoDBClient ddbClient = new AmazonDynamoDBClient(credentialsProvider);
        ddbClient.setRegion(Region.getRegion(Regions.EU_CENTRAL_1));

        DynamoDBMapper mapper = new DynamoDBMapper(ddbClient);

        Condition hashKeyCondition = new Condition()
            .withComparisonOperator(ComparisonOperator.EQ.toString())
            .withAttributeValueList(new AttributeValue().withN("1"));

        Map<String, Condition> keyCondition = new HashMap<String,Condition>();
        keyCondition.put("RestaurantID",hashKeyCondition);

        Map<String, AttributeValue> lastEvaluatedKey = null;
        do{
            QueryRequest queryRequest = new QueryRequest()
                    .withTableName("Menu_Category")
                    .withKeyConditions(keyCondition)
                    .withExclusiveStartKey(lastEvaluatedKey);
            QueryResult queryResult = ddbClient.query(queryRequest);
            for (Map<String, AttributeValue> item : queryResult.getItems()) {
                //Log.i(TAG,"Name = " + item.get("name").getS());
                String i = item.get("name").getS();
                //menu.add(i);
                Log.i(TAG,"Name = " + i);
            }



        }while(lastEvaluatedKey != null);

        return "Executed";
    }

    @Override
    protected void onPostExecute(String result) {
        Log.i(TAG,"onPostExecute = " + result);
    }

    @Override
    protected void onPreExecute() {
        Log.i(TAG,"inPreExecute");
    }

    @Override
    protected void onProgressUpdate(Void... values) {
    }
}

If I comment the line who is adding the String the app doesn't crush. Any idea about this error? This is frustrating.

thanks in advance.

PD: menu is declared private in the activity class.