NavigationConfigForm: Do not allow to delete parents

refs #5600

application/forms/Navigation/NavigationConfigForm.php

@@ -376,6 +376,8 @@ class NavigationConfigForm extends ConfigForm
      * @param   string  $name
      *
      * @return  $this
+     *
+     * @throws  IcingaException     In case the navigation item has still children
      */
     public function delete($name)
     {
@@ -384,6 +386,15 @@ class NavigationConfigForm extends ConfigForm
             throw new NotFoundError('No navigation item called "%s" found', $name);
         }
 
+        $children = $this->getFlattenedChildren($name);
+        if (! empty($children)) {
+            throw new IcingaException(
+                'Unable to delete navigation item "%s". There are other items dependent from it: %s',
+                $name,
+                join(', ', $children)
+            );
+        }
+
         $config->removeSection($name);
         $this->setIniConfig($config);
         return $this;