Row evolution in Custom Report

General questions
1

I want to create a custom report containing all keywords of a campaign with a specific name.
For this purpose I have the following code so far:

 public function getOTHAWCampaign($idSite, $period, $date, $segment = false)
    {
        $data = \Piwik\API\Request::processRequest('Referrers.getCampaigns', array(
            'idSite' => $idSite,
            'period' => $period,
            'date' => $date,
            'segment' => $segment,
            'doNotFetchActions' => true,
        ));
        $data->applyQueuedFilters();

       foreach ($data->getRows() as $visitRow) {
            if ($visitRow->getColumn('label') === "othaw") {
                $id = $visitRow->subtableId;
                var_dump($visitRow);
            }
        }

        return $this->getKeywordsByCampaignId($idSite, $period, $date, $segment, $id);
    }

    public function getKeywordsByCampaignId($idSite, $period, $date, $segment, $idsubtable)
    {
        $result = \Piwik\API\Request::processRequest('Referrers.getKeywordsFromCampaignId', array(
            'idSite' => $idSite,
            'period' => $period,
            'date' => $date,
            'idSubtable' => $idsubtable,
            'segment' => $segment,
            'doNotFetchActions' => false
        ));
        $result->applyQueuedFilters();

        return $result;
    }

Now when I try to open the Row Evolution, I get this error message:

fatal_error
fatal_error1015×207 7.83 KB

This seems to be because of the foreach-statement, if I remove it and change $id to a certain number in the function call getKeywordsByCampaignId(), I can open the Row Evolution without any problems.
But the id can change from day to day, if I have understood correctly.

Does anyone know how I can solve this?