Fix handling default value

app/Http/Controllers/ActivityController.php

@@ -462,8 +462,12 @@ class ActivityController extends Controller
 					do {
 						array_pop($activityStack);
 						$lastStack = end($activityStack);
+						if ($lastStack) {
 							if ($activity->level > $lastStack->level)
 								$lastStackIsNotRight = false;
+						} else {
+							$lastStackIsNotRight = false;
+						}
 					} while ($lastStackIsNotRight);
 				}