class MyThread(QThread):
    
   
    def __init__(self, parent=None):
        QThread.__init__(self, parent)

    countSignal = pyqtSignal(str)
    
    def run(self):
	#Тут вылетает ошибка - "CountThread has no attribute listOfTables"
        selectedTable = self.listOfTables.currentItem().text()
        msg = (selectedTable)
        self.emit(countSignal(msg))
    
    def __del__(self):
        self.exiting = True
        self.wait()


class MForm(QMainWindow):
    
    
    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.ui = Ui_MainForm()
        self.ui.setupUi(self)
	self.ui.pushButton.clicked.connect(self.pushButton_OnClick)
    
    def pushButton_OnClick(self):
	self.t = MyThread()
	self.t.start()
	self.cntThread.countSignal.connect(self.msgBox)

    def msgBox(self, msg):
        QMessageBox.information(self, "Test Program", msg, 1, 0)

if __name__ == "__main__":
    import sys
    app = QApplication(sys.argv)
    myapp = MForm()
    myapp.show()
    sys.exit(app.exec_())

class Thread(QThread):
    def __init__(self, param):
        QThread.__init__(self)
        self.param = param

thread = QThread(1111)