кнопки на второй формеа PyQT не реагируют на нажатие

Я хочу создать приложение, в котором есть два окна. В каждом окне есть по две кнопки. При нажатии одной кнопки на первой форме выводится в консоль слово "Forward", при нажатии второй выводится вторая форма.

И во втором окне при нажатии на одну из кнопок должно просто выводиться сообщение:

import sys
from PyQt5 import QtCore, QtGui, QtWidgets
from First_window import *
from Second_window import *

class MyWin(QtWidgets.QMainWindow):
    def __init__(self,parent=None):
        QtWidgets.QWidget.__init__(self,parent)
        self.ui = Ui_First_window()
        self.ui.setupUi(self)
        self.ui.download_button.clicked.connect(self.handler)
        self.ui.import_button.clicked.connect(self.openWindow)
    def handler(self):
        print("Forward!!!")
    def openWindow(self):
        self.window = QtWidgets.QMainWindow()
        self.ui = Ui_Second_window()
        self.ui.setupUi(self.window)
 #       Second_window.hide()
        self.window.show()

class MySecond(QtWidgets.QMainWindow):
    def __init__(self,parent=MyWin):
        QtWidgets.QWidget.__init__(self,parent)
        self.ui = Ui_Second_window()
        self.ui.setupUi(self)
        self.ui.save_button.clicked.connect(self.handler2)
    def handler2(self):
        print("STOPPP!!!")

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

К сожалению во второй форме кнопка не нажимается...Я пытался сделать следующие изменения

def openWindow(self): 
   self.window = MySecond() 
   self.window.show()

получил ошибку

QtWidgets.QWidget.__init__(self,parent)
TypeError: descriptor '__init__' requires a 'sip.simplewrapper' object but received a 'MySecond'

пытался изменить

QtWidgets.QWidget.__init__(self,parent)

на

QtWidgets.QMainWindow.__init__(self, parent)

и

class MySecond(QtWidgets.QMainWindow):
    def __init__(self,parent=MyWin):

на

class MySecond(QtWidgets.QMainWindow):
    def __init__(self,parent=None):

Никакого прогресса.Как решить данную задачу?

Вот первый модуль

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_First_window(object):
    def setupUi(self, First_window):
        First_window.setObjectName("First_window")
        First_window.resize(605, 480)
        self.centralwidget = QtWidgets.QWidget(First_window)
        self.centralwidget.setObjectName("centralwidget")
        self.horizontalLayoutWidget = QtWidgets.QWidget(self.centralwidget)
        self.horizontalLayoutWidget.setGeometry(QtCore.QRect(90, 80, 431, 101))
        self.horizontalLayoutWidget.setObjectName("horizontalLayoutWidget")
        self.horizontalLayout_2 = QtWidgets.QHBoxLayout(self.horizontalLayoutWidget)
        self.horizontalLayout_2.setContentsMargins(0, 0, 0, 0)
        self.horizontalLayout_2.setObjectName("horizontalLayout_2")
        self.download_button = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.download_button.setObjectName("download_button")
        self.horizontalLayout_2.addWidget(self.download_button)
        spacerItem = QtWidgets.QSpacerItem(40, 20, QtWidgets.QSizePolicy.Expanding, QtWidgets.QSizePolicy.Minimum)
        self.horizontalLayout_2.addItem(spacerItem)
        self.import_button = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.import_button.setObjectName("import_button")
        self.horizontalLayout_2.addWidget(self.import_button)
        self.table_for_data = QtWidgets.QTableWidget(self.centralwidget)
        self.table_for_data.setGeometry(QtCore.QRect(90, 200, 431, 201))
        self.table_for_data.setRowCount(4)
        self.table_for_data.setColumnCount(4)
        self.table_for_data.setObjectName("table_for_data")
        First_window.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(First_window)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 605, 22))
        self.menubar.setObjectName("menubar")
        First_window.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(First_window)
        self.statusbar.setObjectName("statusbar")
        First_window.setStatusBar(self.statusbar)

        self.retranslateUi(First_window)
        QtCore.QMetaObject.connectSlotsByName(First_window)

    def retranslateUi(self, First_window):
        _translate = QtCore.QCoreApplication.translate
        First_window.setWindowTitle(_translate("First_window", "MainWindow"))
        self.download_button.setText(_translate("First_window", "dowload files"))
        self.import_button.setText(_translate("First_window", "import and interpolate"))

Вот вторая форма

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_Second_window(object):
    def setupUi(self, Second_window):
        Second_window.setObjectName("Second_window")
        Second_window.resize(605, 480)
        self.centralwidget = QtWidgets.QWidget(Second_window)
        self.centralwidget.setObjectName("centralwidget")
        self.horizontalLayoutWidget = QtWidgets.QWidget(self.centralwidget)
        self.horizontalLayoutWidget.setGeometry(QtCore.QRect(90, 80, 431, 101))
        self.horizontalLayoutWidget.setObjectName("horizontalLayoutWidget")
        self.horizontalLayout_2 = QtWidgets.QHBoxLayout(self.horizontalLayoutWidget)
        self.horizontalLayout_2.setContentsMargins(0, 0, 0, 0)
        self.horizontalLayout_2.setObjectName("horizontalLayout_2")
        self.save_button = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.save_button.setObjectName("save_button")
        self.horizontalLayout_2.addWidget(self.save_button)
        spacerItem = QtWidgets.QSpacerItem(40, 20, QtWidgets.QSizePolicy.Expanding, QtWidgets.QSizePolicy.Minimum)
        self.horizontalLayout_2.addItem(spacerItem)
        self.import_button = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.import_button.setObjectName("import_button")
        self.horizontalLayout_2.addWidget(self.import_button)
        self.table_for_data = QtWidgets.QTableWidget(self.centralwidget)
        self.table_for_data.setGeometry(QtCore.QRect(90, 200, 431, 201))
        self.table_for_data.setRowCount(4)
        self.table_for_data.setColumnCount(4)
        self.table_for_data.setObjectName("table_for_data")
        Second_window.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(Second_window)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 605, 22))
        self.menubar.setObjectName("menubar")
        Second_window.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(Second_window)
        self.statusbar.setObjectName("statusbar")
        Second_window.setStatusBar(self.statusbar)

        self.retranslateUi(Second_window)
        QtCore.QMetaObject.connectSlotsByName(Second_window)

    def retranslateUi(self, Second_window):
        _translate = QtCore.QCoreApplication.translate
        Second_window.setWindowTitle(_translate("Second_window", "Saving"))
        self.save_button.setText(_translate("Second_window", "save in csv"))
        self.import_button.setText(_translate("Second_window", "import and interpolate"))