I had only this line in my constructor ( as i had posted in the above post). i did not have any other line.
You mean the only line of code in ConnectWidget constructor right?
Does it run if you remove that line as well?
Does it run when this construcotr is empty?
Are you sure you are not trying to use a 'note' or 'm_note' somehwere else in your code?
It would be good if you could post you mail() function.
The problem in your code is not the parts we currently see.
Ofcourse there is also the chance that the QextSerialPort lib is faulty for some reason on your system.