Actually old event handler should use old method name

action.php

@@ -126,7 +126,7 @@ public function editForm(Doku_Event $event, $param){
         // Pass wikitext through POSTs for previewing and saving
         if(isset($_POST['editfigure__new'])) {
             foreach($_POST['editfigure__new'] as $k => $v) {
-                $form->setHiddenField("editfigure__new[$k]", $v);
+                $form->addHidden("editfigure__new[$k]", $v);
             }
         }
     }