PyQt5弹出窗口错误: QWidget::setLayout:正在尝试设置已有布局的QLayout

import sys
from PyQt5.QtWidgets import QApplication, QWidget, QVBoxLayout, QLabel, QPushButton

def show_popup():
    popup = QWidget()
    popup.setWindowTitle("Popup Window")

    layout = QVBoxLayout()
    label = QLabel("This is a popup window.")
    layout.addWidget(label)

    button = QPushButton("Close")
    layout.addWidget(button)

    popup.setLayout(layout)
    popup.show()

if __name__ == "__main__":
    app = QApplication(sys.argv)
    main_window = QWidget()
    main_window.setWindowTitle("Main Window")

    layout = QVBoxLayout()
    label = QLabel("This is the main window.")
    layout.addWidget(label)

    button = QPushButton("Open Popup")
    button.clicked.connect(show_popup)
    layout.addWidget(button)

    main_window.setLayout(layout)
    main_window.show()

    sys.exit(app.exec_())