Sometimes the task is empty (extensions are found that remove the default value from the list).

CRM/DataprocessorSearch/StateMachine/CaseSearch.php

@@ -34,7 +34,7 @@ class CRM_DataprocessorSearch_StateMachine_CaseSearch extends CRM_Core_StateMach
         $this->_pages[$t] = NULL;
       }
     }
-    else {
+    elseif(!empty($task)) {
       $this->_pages[$task] = NULL;
     }