Undefined index: #options in form_process_select()

switch ($instance['widget']['type']) {
        case 'custom_field_widget' :
            $element['custom_select_field'] = array(
                '#type' => 'fieldset',
                '#title' => $element['#title'],
                '#tree' => TRUE,
            );
            $element['custom_select_field']['division'] = array(
                '#type' => 'select',
                '#title' => t('Division'),
                '#default_value' => isset($items[$delta]['district']) ? $items[$delta]['district'] : isset($node->field_division_custom_select['und'][0]['division']) ? $node->field_division_custom_select['und'][0]['division'] : '',
                '#required' => $element['#required'],
                '#id' => 'division-select-list',
                '#attributes' => array('class' => array('selectpicker'), 'title' => t('Thana') ),
                '#DANGEROUS_SKIP_CHECK'=> true,
            );


            $element['custom_select_field']['district'] = array(
                '#type' => 'select',
                '#title' => t('District'),
                '#default_value' => isset($items[$delta]['district']) ? $items[$delta]['district'] : isset($node->field_division_custom_select['und'][0]['district']) ? $node->field_division_custom_select['und'][0]['district'] : '',
                '#required' => $element['#required'],
                '#id' => 'district-select-list',
                '#prefix' => '<div id="dropdown-district-replace">',
                '#suffix' => '</div>',
                '#attributes' => array( 'class' => array('selectpicker'), 'title' => t('Thana') ),
                '#DANGEROUS_SKIP_CHECK'=> true,
            );
            $element['custom_select_field']['thana'] = array(
                '#type' => 'select',
                '#title' => t('Thana'),
                '#default_value' => isset($items[$delta]['thana']) ? $items[$delta]['thana'] : isset($node->field_division_custom_select['und'][0]['thana']) ? $node->field_division_custom_select['und'][0]['thana'] : '',
                '#required' => $element['#required'],
                '#id' => 'thana-select-list',
                '#attributes' => array('class' => array('selectpicker'), 'title' => t('Thana') ),
                '#DANGEROUS_SKIP_CHECK'=> true,
            );
            break;
    }