MVC/ModelRelationField type, fix race condition when referring to ano…

…ther model section not yet loaded.

Since e4d5daf9 we reuse our model object when not referring to another model, which may not yet been completely initialised when referring to a section further in the xml definition. Moving the load action to actionPostLoadingEvent() should assure we do have the related actions loaded. should fix #2389

src/opnsense/mvc/app/models/OPNsense/Base/FieldTypes/ModelRelationField.php

@@ -156,6 +156,13 @@ public function setModel($mdlStructure)
             // set internal key for this node based on sources and filter criteria
             $this->internalCacheKey = md5(serialize($mdlStructure));
         }
+    }
+
+    /**
+     * load model options when initialized
+     */
+    protected function actionPostLoadingEvent()
+    {
         $this->loadModelOptions();
     }