Compile Error: Cannot use isset() on the result of an expression
From documentation:
isset() only works with variables as passing anything else will result in a parse error.
You're not directly passing a variable to isset(). So you need to calculate the value first, assign it to a variable, and then pass that to isset().
For example, what you're doing at the moment is something like:
if(isset($something === false)) { } // throws a parse error, because $something === false is not a variableWhat you need to do instead is:
$something = false;if(isset($something)) { ... }
Why are you doing so much useless code?
Why don't just:
{% if form_action|empty == true %} {% set form_action = '' %} {% endif %}