Error on file upload in structure field with hook, which changes file name

I am using this hook to change uploaded file name to page title and site title + number of file. It looks to work well everywhere, but on frontpage, where I have banner as structure field, I am getting error, that file is not found. And it’s still uploaded with rightly renamed name. I can choose it later, but on upload I see empty file in structure img field.

     'file.create:after' => function ($file) {
          $kirby    = $this;
          $site     = $this->site();
          $page = $file->page();
          $title = $page->title();
          $st    = $site->title();
          $count = $page->files()->count() + 1;	
          $filename = $title . "-" . $st . "-" . $count - 1;
          try {
              } catch(Exception $e) {