Hi René,
The code you mention below is actually the code I've kept in refresh_viewer.scpt. Set up this way, TeXShop doesn't launch. For the time being, this is the solution I've adopted.

René Schwaiger wrote:
Hi Angelo,

On 07 Jan 2015, at 11:18 , Angelo Varlotta <angelovarlotta@gmail.com> wrote:

Hi René,
I tried your suggestions. I've added the snippet, both in addition and in replacement of the previous lines regarding TeXShop in refresh_viewer.scpt, and in both cases at the second and successive uses of "Typeset & View (PDF)", TeXShop opens with no alert dialog.

Jim mentioned that “TeXShop” does not open if he changes `refresh_viewer` in such a way that it always opens “Skim” regardless of the `viewer` argument. According to your description the code for refreshing the PDF in “TeXShop” is never called. Could you please make sure that this is the case by removing the lines that open “TeXShop”. After you made the changes, your copy of `refresh_viewer` should contain the following code:

on run {viewer, path_to_pdf}
    if viewer is "Skim" then
        tell application "Skim"
            revert (documents whose path is path_to_pdf as text)
        end tell
    end if
end run

Does the problem still persist after that?

For me the command

defaults read com.macromates.textmate.preview latexViewer

shows that I'm using Skim, which is my current viewer. I have the latest version of TeXShop (3.48.1) on my computer. In my case I had an extra LaTeX bundle in the Avian directory, but the behavior has persisted even after removing the bundle.

Cheers,
Angelo

Kind regards,
  René