Newbie Coder - "Error at Null" trying to save script

Hi, extremely new to coding in deluge (or coding at all). I'm working on a dependent dropdown function that takes 3 dropdowns and narrows down options based on user entry.

This is what code I have, and every time I try to save it in a workflow I get "error at null". I've looked up the error but documentation on it is non-existent (that I can find anyway), and it seems it happens in multiple circumstances based on prior community posts.

Weakness, Theme, and Key_Lift are the field names

//On User input of Theme
Weakness_Lookup = Weakness;
Key_Lift_Lookup = Key_Lift;
Theme_Lookup = Theme;
if(input.Weakness_Lookup == null && input.Key_Lift_Lookup == null)
{
fet = Exercise_Weaknesses[Theme == input.Theme];
Weakness:ui.add(fet.ID.getall());
Key_Lift:ui.add(fet.ID.getall());
}
if(input.Weakness_Lookup != null && input.Key_Lift_Lookup == null)
{
n = input.Weakness_Lookup;
fet = Exercise_Weaknesses[Theme == input.Theme && Weakness == input.Weakness];
Key_Lift:ui.add(fet.ID.getall());
}
if(input.Weakness_Lookup == null && input.Key_Lift_Lookup != null)
{
k = input.Key_Lift_Lookup;
fet = Exercise_Weaknesses[Theme == input.Theme && Key_Lift == input.Key_Lift];
Weakness:ui.add(fet.ID.getall());
}

What is wrong with the code?

The goal is basically a filter for exercises a strength coach might use when developing a workout program for an athlete or team. The initial idea is to have an output that will populate automatically after the filters are selected. I want to develop it further but need an initial proof.

I have no idea about syntax or anything since I find Zoho's deluge documents to be...well unclear and fragmented at best. VBA for excel is much better documented and I can still barely use that if my life depends on it.