Fixes #6924

plugins/API/Reports/Get.php(70): Warning - array_merge(): Argument 2 is not an array

core/Plugin/Report.php

@@ -405,7 +405,7 @@ class Report
      * default metric translation for this metric using the {@hook Metrics.getDefaultMetricTranslations} event. If you
      * want to overwrite any default metric translation you should overwrite this method, call this parent method to
      * get all default translations and overwrite any custom metric translations.
-     * @return array
+     * @return array|mixed
      * @api
      */
     public function getProcessedMetrics()

plugins/API/Reports/Get.php

@@ -67,7 +67,10 @@ class Get extends Report
     {
         $processedMetrics = array();
         foreach ($this->reportsToMerge as $report) {
-            $processedMetrics = array_merge($processedMetrics, $report->getProcessedMetrics());
+            $reportMetrics = $report->getProcessedMetrics();
+            if (is_array($reportMetrics)) {
+                $processedMetrics = array_merge($processedMetrics, $reportMetrics);
+            }
         }
         return $processedMetrics;
     }