Custom Action Selection to Execute Other Custom Actions

Before I hit a help webinar, I thought I'd pop in note here to see if anyone can assist with my hiccup. 

I am building a custom action with a pick list that, depending upon the selection, will trigger a specific custom action. I had a similar formula I was trying to use as a jumping off point, but am missing something.  (Note, I know just enough html to be superbly frustrating to myself).

I am using a combo of the Catch and If funtions, but can't seem to figure out what to use for my third/false statement at the end of the embedded If function.  In the aforementioned similar formula simply "" was used, but I'm getting an error about a parenthesis.

Here is the formula:

{catch(

if(RequestType='Add New User', GetCustomActionURL('New User Request')),

if(RequestType='Change User Profile', GetCustomActionURL('Change User Profile')),

if(RequestType='Disabled User',GetCustomActionURL('Remove User')),

if(RequestType='Issue',GetCustomActionURL('Issue')), "")))),

NULL)}

The invalid parenthetical input error actually highlights the comma after the Issue Custom Action parentheses.

Any ideas?  Thank you!

Jen