AppleScript for Safari – How to Execute JavaScript on Every Tab/Window

Just because Safari and Chrome are running, doesn't mean that they have a window open. You can either test for open windows as Mark points out or simply issue the reopen command which opens a new window if one does not exist.

tell application "Google Chrome"
    if not (exists window 1) then reopen
    -- insert your code 
end tell

tell application "Safari"
    if not (exists document 1) then reopen
    -- insert your code 
end tell

EDIT

tell application "Safari"
    if not (exists document 1) then return
    set theURL to URL of current tab of window 1
    close current tab of window 1
end tell

tell application "Google Chrome"
    if not (exists window 1) then reopen
    set URL of active tab of window 1 to theURL
end tell

I have created a bundled script that saves the current tab to the desktop as a webarchive. Credit to newzealandpaul for his webarchiver shell command that powers this script. You can tweak the code to fit your specific needs, but this should do what that code in your question is trying to do.

Here is the bundled script: SaveWebarchive

This works with the webarchiver command bundled;

set fileName to "foo"

tell application "Safari" to set targetURL to (URL of document 1) as string
set commandPath to POSIX path of (path to resource "webarchiver.command")
do shell script "" & commandPath & " -url " & targetURL & " -output ~/Desktop/" & fileName & ".webarchive"

Note that the downloaded script may have the Script Editor interface showing the Log over the entire window.