GenericTable: prevent passing NULL to strtotime

models/GenericTable.php

@@ -234,7 +234,9 @@ class GenericTable
 				else
 					$pattern = $options['data']['pattern'];
 
-				if (!is_numeric($rowData[$options['data']['timestamp']]))
+				if (!isset($rowData[$options['data']['timestamp']]))
+					$timestamp = 0;
+				elseif (!is_numeric($rowData[$options['data']['timestamp']]))
 					$timestamp = strtotime($rowData[$options['data']['timestamp']]);
 				else
 					$timestamp = (int) $rowData[$options['data']['timestamp']];