Use Save As button to create unique file name from form fields

Hello,

I have a form that I've added a button to that needs to pull from form fields and save the file with those field names to create a unique file name.

I've read the old post on this topic:

Solved: Save PDF File Name as Field Name in PDF - Adobe Community - 7231510

However, it's not working for me. 

Steps I've taken:

1. Created JS file in the user directory for the trusted function C:\Users\Username\AppData\Roaming\Adobe\Acrobat\Privileged\DC\JavaScripts (this is for work and I don't have administrator access to save in the JS folder). 

2. Copied the JS file into the folder where the PDF lives

3. File contains script:

var mySaveAs = app.trustedFunction(function(oDoc, cPath, cFlName) {
FILE PATH REMOVED PER USER'S REQUEST

Sandbox/22178_Fillable 1_Out - 5 Sandbox Save to Folder.pdf/
cPath = cPath.replace(/([^/])$/, "$1/");
try {
app.beginPriv();
oDoc.saveAs(cPath + cFlName);
app.endPriv();
} catch (e) {
app.alert("Error During Save - " + e);
}
}
);

4. Added button to my form

5. Set button to run a Javascript with the following script:

// determine the directory path for the current document
var directory = this.path.substring(0, this.path.lastIndexOf('/') +1);
if (typeof(mySaveAs) == "function") {
mySaveAs(this, directory, this.getField("FirstName").value + " " + this.getField("LastName").value + " " + this.getField("DateofCalldd-Mon-yyyy").value + ".pdf");
} else {
app.alert("Missing Save Function. Please contact forms administrator ");
}

When I click the button, I get the following error message in the debugger

TypeError: this.getField(...) is null
4:Field:Mouse Up

Thanks for any help on this!