Bugfix: Fix a failure when startcolumn is not set in the datepicker instance.

Author: mm-poustini

classes/wunderbyte_table.php

@@ -1687,7 +1687,9 @@ public function apply_filter(string $filter, string &$searchtext = '') {
                             is_object($category->datepicker)
                         ) {
                             foreach ($category->datepicker->datepickers as $dp) {
-                                $allowedfilters[] = $dp->startcolumn;
+                                if (!empty($dp->startcolumn)) {
+                                    $allowedfilters[] = $dp->startcolumn;
+                                }
                                 if (!empty($dp->endcolumn)) {
                                     $allowedfilters[] = $dp->endcolumn;
                                 }