Counting number of documents in portfolio in acrobat DC pro

I used acrobat to export several thousand emails to a pdf portfolio.  (Worked great by the way and I love the way attachments to email stay attached as links to pdf.)

I now need to count the number of documents in the portfolio.

Found this question with an answer in the forum here: How To Display Acrobat Portfolio File Count in DC? 

Answer involved using the javascript which I'd never done.  I opened the debug console and followed the instructions in the post.  I get an error.  So, (1) is there any way to do this without using javasript or, if not, (2)what am I doing wrong. 

What I did: Opened console and typed:

app.alert("This portfolio has " + this.data.Objects.length + " files attached to it.",3);

When I select the text given and use Cntrl-Enter, get this error: "TypeError: this.dataObjects is null
1:Console:Exec
undefined

No document within portfolio is open, though I am seeing preview in bottom pane (which I don't know how to get rid of).