问AppleScript中的Javascript 'missing value‘(用于在Safari中单击按钮)

EN

set buttontext to "Add Option"
set buttonloc to 1

tell application "Safari"
    activate
    tell window 1
        do JavaScript "var buttonTags = document.getElementsByTagName(\"button\"); 
        var searchText = \"" & buttontext & "\"; 
        var found; 
        for (var i = 0; i < buttonTags.length; i++) 
        {if (buttonTags[i].textContent == searchText) {
        found = buttonTags[i]; break;
        }
        }
        buttonTags[" & buttonloc & "].click();"
    end tell
end tell