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I need some help in my current problem.

I have a window with three buttons. When pressing two of them ("GraphicButton" and "QuestionButton") a second and third window shall open. This works fine. The third button "ImportButton" is for populating a table widget in the mainwindow with a dataframe from excel.
When I press the "ImportButton" first everything works fine.

BUT when I press one of the other buttons first to open the other windows and THEN press the "ImportButton", the mainwindow and the other ones will close.

I could use some advice here.
Here is my code for main.py:

import sys
import pandas as pd
import os
from qtpy import QtWidgets, QtCore, QtGui

from ui.mainwindow import Ui_MainWindow
from ui.graphwindow import Ui_GraphWindow
from ui.questionwindow import Ui_QuestionWindow

app = QtWidgets.QApplication(sys.argv)

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

 self.ui = Ui_MainWindow()
 self.ui.setupUi(self)
 self.setWindowTitle("Test")

 self.ui.ImportButton.clicked.connect(self.import_clicked)
 self.ui.GraphicButton.clicked.connect(self.graphic_clicked)
 self.ui.QuestionButton.clicked.connect(self.question_clicked)

 def import_clicked(self):
 self.ui.RefWidget.setSortingEnabled(False)
 self.ui.LubWidget.setSortingEnabled(False)
 self.ui.MixWidget.setSortingEnabled(False)

 self.ui.RefWidget.setRowCount(0)
 self.ui.LubWidget.setRowCount(0)
 self.ui.MixWidget.setRowCount(0)

 if self.ui.RefBoxC.currentIndex() == 0:
 A = os.path.join(os.path.dirname(__file__),"./Datenbank/CO2.xlsx")
 df = pd.read_excel(A)

 self.ui.RefWidget.setColumnCount(len(df.columns))
 self.ui.RefWidget.setRowCount(len(df.index))

 for i in range(len(df.index)):
 for j in range(len(df.columns)):
 self.ui.RefWidget.setItem(i, j, QtWidgets.QTableWidgetItem(str(df.iat[i, j])))

 self.ui.RefWidget.resizeColumnsToContents()
 self.ui.RefWidget.resizeRowsToContents()
 else:
 A = os.path.join(os.path.dirname(__file__), "./Datenbank/PlatzhalterRef.xlsx")
 df = pd.read_excel(A)

 self.ui.RefWidget.setColumnCount(len(df.columns))
 self.ui.RefWidget.setRowCount(len(df.index))

 for i in range(len(df.index)):
 for j in range(len(df.columns)):
 self.ui.RefWidget.setItem(i, j, QtWidgets.QTableWidgetItem(str(df.iat[i, j])))

 self.ui.RefWidget.resizeColumnsToContents()
 self.ui.RefWidget.resizeRowsToContents()

 self.ui.RefWidget.setSortingEnabled(True)
 self.ui.LubWidget.setSortingEnabled(True)
 self.ui.MixWidget.setSortingEnabled(True)

 def graphic_clicked(self):
 self.Gwindow = QtWidgets.QMainWindow()
 self.ui = Ui_GraphWindow()
 self.ui.setupUi(self.Gwindow)
 self.Gwindow.show()

 def question_clicked(self):
 self.Questwindow = QtWidgets.QMainWindow()
 self.ui = Ui_QuestionWindow()
 self.ui.setupUi(self.Questwindow)
 self.Questwindow.show()

window = MainWindow()
window.show()

sys.exit(app.exec_())

Disclaimer: the solution provided by this answer has not been tested and may be incomplete.

When you click either the GraphicButton or QuestionButton,

self.ui = Ui_GraphWindow()

or

self.ui = Ui_QuestionWindow()

This will modify self.ui. When the ImportButton is clicked, self.ui will refer to the other windows. This, I believe, is unintentional.

When the commands in import_clicked() are run, there may be references to undefined classes, widgets, and whatnot.

self.ui.RefWidget.setSortingEnabled(False)

I'm guessing RefWidget is not defined in Ui_QuestionWindow or Ui_GraphWindow.

To solve this, try adding

def import_clicked(self):
 self.ui = Ui_MainWindow() # set self.ui to refer to a MainWindow
 # ...

and see if it works. (Warning: a new MainWindow may be created and the old MainWindow be left forsaken. Again, I haven't tested this to see if it does create a new MainWindow but my intuition tells me that it does.)

EDIT: Above suggestion in strikethrough did not work for OP.

If further trouble and chaos ensues, consider having three separate member variables instead of a single self.ui.

def __init__(self, parent = None):
 super().__init__(parent)

 self.uiMain = Ui_MainWindow() # new member variable: self.uiMain
 # ...

def import_clicked(self): 

 self.uiMain.RefWidget.setSortingEnabled(False)
 self.uiMain.LubWidget.setSortingEnabled(False)
 self.uiMain.MixWidget.setSortingEnabled(False)
 # ...

def graphic_clicked(self):
 self.Gwindow = QtWidgets.QMainWindow()
 self.uiGraphic = Ui_GraphWindow() # new member variable: self.uiGraphic
 # ...

def question_clicked(self):
 self.Questwindow = QtWidgets.QMainWindow()
 self.uiQuestion = Ui_QuestionWindow() # new member variable: self.uiQuestion
 # ...