fix(management) Avoid calling update on NULL

Management dialogs tend to close themselves from readPost method, so
 after calling readPost we need to check the dialog is still there before
 calling update.

issue #6072

include/management/class_management.inc

@@ -506,7 +506,12 @@ class management implements FusionDirectoryDialog
     } elseif (is_object($this->dialogObject)) {
       try {
         $this->dialogObject->readPost();
-        $this->dialogObject->update();
+        if (is_object($this->dialogObject)) {
+          /* Check again as readPost might close it */
+          if (!$this->dialogObject->update()) {
+            $this->closeDialogs();
+          }
+        }
       } catch (FusionDirectoryException $e) {
         $error = new FusionDirectoryError(htmlescape($e->getMessage()), 0, $e);
         $error->display();

include/management/class_templateDialog.inc

@@ -96,7 +96,7 @@ class templateDialog implements FusionDirectoryDialog
 
   public function update(): bool
   {
-    return TRUE;
+    return !$this->closed;
   }
 
   function setNextTarget ($target)
@@ -131,6 +131,6 @@ class templateDialog implements FusionDirectoryDialog
   protected function handleCancel ()
   {
     $this->management->remove_lock();
-    $this->management->closeDialogs();
+    $this->closed = TRUE;
   }
 }