Some little improvements

This module is great and really required. It would be very cool, if this could be an option integrated in entityreference in the future or become a full project release.

Now some improvements I made when using this module:

function entityreference_trim_field_widget_form_alter(&$element, &$form_state, $context) {
  if (isset($element['target_id'])) {
    $instance = field_info_instance($element['target_id']['#entity_type'], $element['target_id']['#field_name'], $element['target_id']['#bundle']);
    if ($instance['widget']['type'] == 'entityreference_autocomplete') {
      drupal_add_js(drupal_get_path('module', 'entityreference_trim') . '/js/entityreference_trim.js');
      if ($element['target_id']['#default_value']) {
        preg_match("/.+\((\d+)\)/", $element['target_id']['#default_value'], $matches);
        $element['target_id']['#hidden_target_id'] = $matches[1];
        $element['target_id']['#default_value'] = trim(str_replace('(' . $matches[1] . ')', '', $element['target_id']['#default_value']));
      }
// ######## change: I've added this because otherwise the module does not work for fields that don't have a default value set ##############
      else {
        $element['target_id']['#hidden_target_id'] = $element['target_id']['#default_value'];
      }
      $element['target_id']['#element_validate'][0] = '_entityreference_trim_autocomplete_validate';
    }
  }
}

And

/**
 *  validation callback for entityreference_autocomplete.
 */
function _entityreference_trim_autocomplete_validate($element, &$form_state, $form) {
  $elementId = $element['#id'];
  // if a value was entered into the autocomplete...
  $value = '';
  $target_id = isset($form_state['input'][$elementId]) ? drupal_explode_tags($form_state['input'][$elementId]) : 0;
  if (!empty($element['#value'])) {
    $value = $target_id ? $target_id[0] : $element['#hidden_target_id'];
  }


// ######### change: Verify that the selected entity value truely exists!! Use the logic fom entityreference.module for that #########
  // Check if the value of the field truely exists!
  $field = field_info_field($element['#field_name']);
  $handler = entityreference_get_selection_handler($field);
  $field_name = $element['#field_name'];
  $field = field_info_field($field_name);
  $instance = field_info_instance($element['#entity_type'], $field_name, $element['#bundle']);
  $handler = entityreference_get_selection_handler($field, $instance);
  $handler->validateAutocompleteInput($element['#value'], $element, $form_state, $form);

  // update the value of this element so the field can validate the product IDs.
  form_set_value($element, $value, $form_state);
}

Please have a look and feel free to integrate this, if you aggree it makes sense.

Both things work great in my case.