PyQt5 Function without arguments giving error: argument 1 has unexpected type 'NoneType'

I am creating a highly generic template for PyQt5 applications. Without all the comments, here is the code so far. However, executing this code gives the following error:

self.button_widget.clicked.connect(self.functionX)
TypeError: argument 1 has unexpected type 'NoneType'

I am confused about this because the function takes no arguments other than self. What is going on here?

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
import sys

class mainWindow(QWidget):
    def __init__(self):
        super(mainWindow, self).__init__()

        self.title = 'Window Title'
        self.left = 100
        self.top = 100
        self.width = 750
        self.height = 500
        self.setWindowTitle(self.title)
        self.setGeometry(self.left, self.top, self.width, self.height)

        self.do_something()

        self.button_widget = QPushButton()
        self.layout = QGridLayout()
        self.layout.addWidget(self.button_widget, 0, 0)
        self.layout.addWidget(self.widgetX, 1, 0, 1, 3) # row, col, row_span, col_span
        self.setLayout(self.layout)

        self.button_widget.clicked.connect(self.functionX)

    def do_something(self):
        self.widgetX = ClassX()
        self.functionX = self.widgetX.functionX()
        self.variableX = self.widgetX.variableX
	
class ClassX(QLabel):
    def __init__(self):
        super(ClassX, self).__init__()

        self.setFixedHeight(40)
        self.setMinimumWidth(600)
        self.setText('I am a Custom QLabel')
        self.variableX = 'X'

    def functionX(self):
        self.setText('I can change text')


if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = mainWindow()
    ex.show()
    sys.exit(app.exec_())

I originally tried using generic arguments like args=None, but this resulted in the same issue. I believe something is going on with using self in both the mainWindow class and the ClassX class. This is NOT the same question as others that ask about this TypeError being given when using a function that takes an argument (like a string or other data type, not self).