explo new storm with more volatile

library/Class/SessionActivity.php

@@ -481,14 +481,17 @@ class Class_SessionActivity extends Storm_Model_Abstract {
   }
 
 
-  /** @return string */
   public function getLibelleActivity() {
-    return $this->getActivity()->getLibelle();
+    return ($activity = $this->getActivity())
+      ? ($activity->getLibelle() ?? '')
+      : '';
   }
 
 
   public function getActivityDescription() {
-    return $this->getActivity()->getDescription();
+    return ($activity = $this->getActivity())
+            ? $activity->getDescription()
+            : '';
   }