The red screen of death help (unexpected null value)

So I am getting the following "red screen of death"

I have a custom action I got from the market place with the following code. It is used for users to select a month

I also have a custom function to convert the result of the "showMonthPicker" into a string that I can display on the app.

Why am I getting the red screen of death? How can I fix this? Thanks everyone!!!