Fatal Error after deleting a node or module

I've replaced slideshow images with my own and everything works fine until I delete a module or a page, after that the images get deleted from the server (I checked via FTP) and the front page shows the following error

Fatal error: Call to a member function getFileUri() on a non-object in /themes/drupal8_custom_theme/drupal8_custom_theme.theme on line 136

The error is not fixed by reuploading files to the server via FTP, I have to upload them again via theme settings

Admin pages are working fine though