无法在QWidget上绘制NavigationToolbar

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QVBoxLayout, QWidget
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure
from matplotlib.backends.backend_qt5agg import NavigationToolbar2QT as NavigationToolbar

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        # 创建一个Figure对象和一个布局
        self.figure = Figure()
        self.canvas = FigureCanvas(self.figure)
        self.toolbar = NavigationToolbar(self.canvas, self)

        # 创建一个QWidget作为主窗口的中心部件
        widget = QWidget()
        layout = QVBoxLayout(widget)
        layout.addWidget(self.toolbar)
        layout.addWidget(self.canvas)
        self.setCentralWidget(widget)

        # 调用绘图函数
        self.plot()

    def plot(self):
        # 在Figure对象上进行绘图操作
        ax = self.figure.add_subplot(111)
        ax.plot([1, 2, 3, 4, 5], [1, 2, 3, 4, 5])

        # 更新绘图
        self.canvas.draw()

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