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);