Collect used document fonts and export them to folder

And you could check if the active document was ever saved.

If not, exit, because there is no fullName of the document.

if(app.documents.length == 0){ exit() };
if(!app.documents[0].saved){ exit() };

Put both lines at the beginning of your script.

However, it would be better to wrap all the code into a self-executing anonymous function like below.

Simply use return instead of exit() inside this function.

( function()
{
    // No document open:
    if(app.documents.length == 0){ return };
    // Document was never saved:
    if(!app.documents[0].saved){ return };
    var myDocument = app.activeDocument;
    var myFonts = myDocument.fonts.everyItem().getElements();
    
    var len = myFonts.length;
    var cFile, cName, outFile, i ;
    
    var movefolder = File( myDocument.fullName).parent+"/"+ "Document fonts" ;
    movefolder = Folder( movefolder );
    if (!movefolder.exists){ movefolder.create() };
    for(i=0;i<len;i++)
    {  
        cFile = File( myFonts.location ); 
        if( !cFile.exists ){ continue } ; 
        cName = cFile.name; 
        outFile = File(movefolder + "/"+ cName); 
        cFile.copy(outFile);
        
    } 
}() )

Regards,
Uwe