Connection is invalid error when running Applescript and Safari

Solution 1:

You need to swap the position of the activate line of code with the if not (exists document 1) then reopen line of code.

Example:

tell application "Safari"
    if not (exists document 1) then reopen
    activate
    tell front document
        set URL to "http://example.com"
        repeat until length of (source as text) is not 0
            delay 0.5
        end repeat
        do javascript "document.body.innerText"
    end tell
end tell

The screenshot of the Script Editor, below, was run with Safari open and without any windows opened.

First I had to check "[√] Show Develop menu in menu bar" (Safari > Preferences… > Advanced) and then on the Develop menu click "Allow JavaScript from Apple Events". Next I copied and pasted your AppleScript code in Script Editor and clicked the Run the Script button and it ran without error and returned the text of the http://example.com web page. However Safari was closed when I ran the script the first time.

After reading your question again, I was able to reproduce the error by having Safari open without any windows open. I then looked more throughly at your code and noticed activate was before if not (exists document 1) then reopen and activate needs to be after it to avoid the error.