import sys
from PyQt5.QtWidgets import QApplication, QWidget, QMainWindow, QPushButton
from PyQt5.QtCore import QCoreApplication
from PyQt5.QtGui import QIcon


class MyApp(QMainWindow, QWidget):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        self.setWindowTitle("테스트창")
        self.setFixedSize(800, 300)
        self.move(500, 500)
        self.setWindowIcon(QIcon("web.png"))

        btn = QPushButton("종료 버튼", self)
        btn.move(50, 50)
        btn.resize(100, 50)
        btn.clicked.connect(QCoreApplication.instance().quit)

        self.statusBar().showMessage("테스트창 진입 완료.")

        self.show()


def main():
    app = QApplication(sys.argv)
    ex = MyApp()
    sys.exit(app.exec_())


if __name__ == "__main__":
    main()

여기서 class MyApp()선언 부분에서 class MyApp(QWidget, QMainWindow)으로 상속을 받으면 오류가 뜨고 class MyApp(QMainWindow, QWiget)으로 상속 받으면 괜찮은데 왜 그런 건가요?