You have to use this syntax in the field in the show region:
<?php echo(isset($_GET['action'])$_GET['action']:""); ?>
You can only refer to:
<?php echo($_GET['action']); ?>
if there will always be an action in the url, which there won't on your page.
It is a valid input, so our extension won't catch and refuse to use it, but it doesn't know that the variable you are referencing won't always exist. Since it won't you have to use code that checks if it exists before using it like:
<?php echo(isset($_GET['action'])$_GET['action']:""); ?>
In web assist bindings we will add that automatically, but when you use the Dremweaver bindings or hand code, you need to add it manually if the variable might not exist.