public function form( $instance ) {

  /* Set up some default widget settings. */
  $defaults = array( 'title' => 'Classes by Category' );
  $instance = wp_parse_args( (array) $instance, $defaults ); ?>

  <!-- Widget Title -->
  <p>
    <label for="<?php echo $this->get_field_id( 'title' ); ?>">Title:</label>
    <input type="text" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" value="<?php echo $instance['title']; ?>" style="width:100%;" />
  </p>

  <!-- Categories Dropdown -->
  <p>
    <label for="<?php echo $this->get_field_id( 'cs-category' ); ?>">Choose Class Category:</label>

    <?php
      $dropdown_args = array(
        'taxonomy' => 'cs_categories',
        'id' => $this->get_field_id( 'cs-category' ),
        'show_option_none' => __( 'Select category' ),
        'hide_empty' => true,
        'hierarchical' => true,
        'depth' => 2,
        'echo' => 0,
        'selected' => $instance[ 'cs-category' ],
        'class' => 'widefat'
      );

      echo wp_dropdown_categories( $dropdown_args );
    ?>
  </p>

  <?php
}

public function update( $new_instance, $old_instance ) {
  $instance = $old_instance;
  $instance[ 'title' ] = strip_tags( $new_instance[ 'title' ] );
  $instance[ 'cs-category' ] = filter_var( $new_instance[ 'cs-category' ], FILTER_SANITIZE_NUMBER_INT );
  return $instance;
}