WordPress custom widget undefined variable notice

You can you wp_parse_args to merge $instance with a default array. So, you would start you form function not with extract($instance) but with

$defaults = array( 
    'title'         => 'Your title',
    'text'          => '...',
    'image_url'     => '...'
    'textarea'      => '...'
    );
$instance = wp_parse_args((array) $instance, $defaults);