Why is PyQt5 not displaying button?

Question

When the menu class is inheriting from QWidgets class, the button is displayed, however, when is inheriting from QMainWindow the same button doesnt display. I've searched everywhere but couldn't find a explanation.

from PyQt5.QtWidgets import QMainWindow, QFormLayout, QPushButton, QApplication, QWidget
import sys


class Menu(QMainWindow):
    """
    Create menu object
    """

    def __init__(self, parent=None):
        """
        Initialize main menu
        """
        super().__init__(parent)
        self.file = None
        self.fields_create()
        self.create_table()

    def fields_create(self):
        """
        Create buttons and LineEdits
        """
        # Settings Button
        self.settings_btn = QPushButton()
        self.settings_btn.setText("Settings")
        self.settings_btn.clicked.connect(self.settings_click)

    def create_table(self):
        """
        create and display table
        """
        self.flo = QFormLayout()
        self.flo.addRow(self.settings_btn)
        self.setLayout(self.flo)

    def settings_click(self):
        print("clicked")


if __name__ == "__main__":
    app = QApplication(sys.argv)
    win = Menu()
    win.show()
    sys.exit(app.exec_())

Answer 1

try to replace :

self.settings_btn = QPushButton()

with

self.settings_btn = QPushButton(self)