PHP error when going to /admin/config/content/formats/xxx

When you navigate to /admin/config/changes/formats/xxx (where xxx is the name of an input format), the settings callback provided by hook_filter_info() is invoked.

Geshifilter's is:

function geshifilter_filter_settings_callback($form, &$form_state, $filter, $format, $defaults, $filters) {
  require_once drupal_get_path('module', 'geshifilter') .'/geshifilter.admin.inc';
  return _geshifilter_filter_settings($form, $form_state, $filter, $format, $defaults, $filters);
}

The $format variable is an object with standard properties. $format->format represents the machine name of the format.

This variable is passed to a helper to fetch geshifilter settings, which calls a helper, which calls another helper...which eventually calls variable_get using the machine name of the format:

function _geshifilter_filter_settings($form, &$form_state, $filter, $format, $defaults, $filters) {
  ...
  $settings['general_tags'] = _geshifilter_general_highlight_tags_settings($format);



function _geshifilter_general_highlight_tags_settings($format = NULL) {
  ...
  '#default_value' => _geshifilter_tag_styles($format),



function _geshifilter_tag_styles($format = NULL) {
  ...
  return variable_get("geshifilter_tag_styles_{$format}", _geshifilter_tag_styles());

Throughout, $format is an object, so variable_get("geshifilter_tag_styles_{$format}" will fail.
The attached patch invokes the helpers with $passed_format instead (which is the machine name of the format).

An alternative approach might be to simply change the variable_get to variable_get("geshifilter_tag_styles_{$format->format}".