Error Undefined variable

Hi!

Thanks for reporting the notice again; it’ll be fixed in the next release. In the meantime, just take a look at this other thread for applying the solution yourself.

Regarding the issue with the twenty fifteen theme, that’s not possible. I use that theme in my development installation and featured images work just fine. What’s amiss in your case?

Thanks for replying.
Sorry for not describing particularly. It’s Previous/Next Posts section at the end of every posts in twentyfifteen theme

Hi!

You’re absolutely right; in the Prev/Next posts, featured image is not visible. In order to overcome this issue, add the following function in theme’s functions.php file (or, if you have a plugin where you add your own customization, add it there):

function nelioefi_twentyfifteen_post_nav_background() {
  if ( ! is_single() ) {
    return;
  }

  $previous = ( is_attachment() ) ? get_post( get_post()->post_parent ) : get_adjacent_post( false, '', true );
  $next     = get_adjacent_post( false, '', false );
  $css      = '';

  if ( is_attachment() && 'attachment' == $previous->post_type ) {
    return;
  }

  if ( $previous &&  uses_nelioefi( $previous->ID ) ) {
    $css .= '
      .post-navigation .nav-previous { background-image: url(' . esc_url( nelioefi_get_thumbnail_src( $previous->ID ) ) . '); }
    ';
  }

  if ( $next && uses_nelioefi( $next->ID ) ) {
    $css .= '
      .post-navigation .nav-next { background-image: url(' . esc_url( nelioefi_get_thumbnail_src( $next->ID ) ) . '); border-top: 0; }
    ';
  }

  wp_add_inline_style( 'twentyfifteen-style', $css );
}
add_action( 'wp_enqueue_scripts', 'nelioefi_twentyfifteen_post_nav_background', 99 );

Best,