Bugfix: Also catch null in date field controller.

Author: bernhard-wunderbyte

classes/local/customfield/field/date/wbt_field_controller.php

@@ -41,12 +41,16 @@ class wbt_field_controller extends field_controller implements wbt_field_control
     /**
      * Get the actual string value of the customfield.
      *
-     * @param string|array $key
+     * @param string|array|int|null $key
      * @param bool $formatstring
      * @param bool $keyisencoded
      * @return string the string value
      */
-    public function get_option_value_by_key(string|array $key, bool $formatstring = true, bool $keyisencoded = false): string {
+    public function get_option_value_by_key(
+        string|array|int|null $key,
+        bool $formatstring = true,
+        bool $keyisencoded = false
+    ): string {
         if (empty($key) || $key == 0) {
             return '';
         }