Call another field inside preprocess_views_view_field
Ok,
I solved it like this:


function THEMENAME_preprocess_views_view_field(&$variables) {
  $view = $variables['view'];
  $field = $variables['field'];
  $rows = $variables['rows'];

  if ($view->id() == 'VIEW_NAME' && $view->current_display == 'DISPLAY_NAME') {
    if ($field->field == 'FIELD_NAME') {      
      foreach ($view->field as $fieldId => $field) {
        if ($fieldId === 'ANOTHER_FIELD_NAME_IN_VIEW') {
          $my_output = $view->style_plugin->getFieldValue($row->index, $fieldId);
        }
      }
      $variables['output'] = $my_output;
    }
  }
}