Multiselect FAPI not working inside hook_field_widget_settings_form()

function ti_editorial_recommendations_engine_field_widget_settings_form($field, $instance) {
$form['multiselector'] = array(
'#type' => 'multiselect',
'#title' => t('Here is the multiselect'),
'#options' => array(
'title' => t('Titles only'),
'teaser' => t('Titles plus teaser'),
'fulltext' => t('Full text'),
'1' => t('Option 1'),
'2' => t('Option 2'),
'3' => t('Option 3'),
'4' => t('Option 4'),
'5' => t('Option 5'),
'6' => t('Option 6'),
'7' => t('Option 7'),
'8' => t('Option 8'),
'9' => t('Option 9'),
'10' => t('Option 10'),
'11' => t('Option 11'),
'12' => t('Option 12'),
),
'#default_value' => _multiselect_fapi_example_options(),
'#multiple' => TRUE,
'#size' => 15,
'#required' => TRUE,
);

return $form;
}
Multiselect widget is rendering but ADD REMOVE options are not working from left right.