Exercise: fix iId to iid in the signature plugin to make it work again - refs BT#19114

plugin/exercise_signature/lib/ExerciseSignature.php

@@ -28,13 +28,13 @@ class ExerciseSignaturePlugin extends Plugin
 
     public static function exerciseHasSignatureActivated(Exercise $exercise)
     {
-        if (empty($exercise->iId)) {
+        if (empty($exercise->iid)) {
             return false;
         }
 
         if ('true' === api_get_plugin_setting('exercise_signature', 'tool_enable')) {
             $extraFieldValue = new ExtraFieldValue('exercise');
-            $result = $extraFieldValue->get_values_by_handler_and_field_variable($exercise->iId, 'signature_activated');
+            $result = $extraFieldValue->get_values_by_handler_and_field_variable($exercise->iid, 'signature_activated');
             if ($result && isset($result['value']) && 1 === (int) $result['value']) {
                 return true;
             }