matplotlib basemap colorbar exception : Given element not contained in the stack

I am new to matplotlib, I got exception

Traceback (most recent call last):
  File "C:\Users\Oscar\Documents\ConsumerSoftwareProject\Plot\pyqt��������.py", line 44, in handle_clicked_1
    self.update_plot(
  File "C:\Users\Oscar\Documents\ConsumerSoftwareProject\Plot\pyqt��������.py", line 109, in update_plot
    cb = m.colorbar()
  File "C:\Users\Oscar\AppData\Local\Programs\Python\Python39\lib\site-packages\mpl_toolkits\basemap\__init__.py", line 4647, in colorbar
    fig.sca(ax) # reset parent axes as current axes.
  File "C:\Users\Oscar\AppData\Local\Programs\Python\Python39\lib\site-packages\matplotlib\figure.py", line 1535, in sca
    self._axstack.bubble(a)
  File "C:\Users\Oscar\AppData\Local\Programs\Python\Python39\lib\site-packages\matplotlib\figure.py", line 82, in bubble
    return super().bubble(self._entry_from_axes(a))
  File "C:\Users\Oscar\AppData\Local\Programs\Python\Python39\lib\site-packages\matplotlib\cbook\__init__.py", line 638, in bubble
    raise ValueError('Given element not contained in the stack')
ValueError: Given element not contained in the stack

with the following self-contained code, so how to fix the problem?

import sys

from PyQt5 import QtWidgets

from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt5agg import NavigationToolbar2QT as NavigationToolbar
from matplotlib.figure import Figure

import numpy as np

from mpl_toolkits.basemap import Basemap
from matplotlib import cm


class MainWindow(QtWidgets.QMainWindow):
        def __init__(self, parent=None):
                super().__init__(parent)

                button1 = QtWidgets.QPushButton("button1")
                button2 = QtWidgets.QPushButton("button2")

                self.map_canvas = FigureCanvas(Figure(figsize=(20, 20)))
                self.map_toolbar = NavigationToolbar(self.map_canvas, self)

                central_widget = QtWidgets.QWidget()
                self.setCentralWidget(central_widget)
                lay = QtWidgets.QGridLayout(central_widget)
                lay.addWidget(button1, 0, 0)
                lay.addWidget(button2, 0, 1)
                lay.addWidget(self.map_canvas, 1, 0, 1, 2)

                self.addToolBar(self.map_toolbar)

                button1.clicked.connect(self.update_plot)
                button2.clicked.connect(self.update_plot)

                self.ax = self.map_canvas.figure.subplots()
                self.ax.set_axis_off()
                # help(self.map_canvas.figure)

        def update_plot(self):
                self.ax.clear()

                m = Basemap(width=12000000, height=9000000, projection='lcc', resolution='c', lat_0=19., lon_0=73., ax=self.ax,)
                m.drawcoastlines(linewidth=0.25)  # I added color='red', lw=2.0

                x, y, z = np.random.rand(3, 1000000)
                x *= 12e6
                y *= 9e6
                z *= 20000

                gridsize = 100
                m.hexbin(x, y, C=z, gridsize=gridsize, cmap=cm.YlGnBu)

                cb = m.colorbar()

                # self.map_canvas.figure.title('SAT on January 1')
                # self.map_canvas.figure.colorbar(self.ax)
                self.map_canvas.draw()


if __name__ == "__main__":

        app = QtWidgets.QApplication.instance()
        if app is None:
                app = QtWidgets.QApplication(sys.argv)

        window = MainWindow()
        window.show()

        sys.exit(app.exec_())