I have the following applescript working under Lion, but it returns this Error: -10000. Mail got an error: AppleEvent handler failed. error under Yosemite.

try
tell application "Mail"
    set theViewer to front message viewer
    set theMsg to messages of theViewer
    set msgList to {}
    repeat with thisMsg in theMsg
        if read status of thisMsg is false and flagged status of thisMsg is false then
            set the end of msgList to thisMsg
        end if
    end repeat
    if msgList is {} then
        display dialog "There are no read messages in this mailbox."
    else
        tell theViewer to set properties to {visible messages:msgList}
    end if
end tell
on error the errMsg number the errNmb
if the errNmb is not -128 then
    set the errTxt to "Error: " & the errNmb & ". " & the errMsg
    display dialog the errTxt buttons {"Cancel"} default button 1
else
    error number -128
end if

end try

Any help would be appreciated.