free(): invalid pointer

[nyanmn]

I have the following code and PyQT5 is used for display.

import sys
from PyQt5.QtWidgets import (QApplication, QWidget, QPushButton, QGridLayout, QLabel)
import mmcv

#Display images
class Example(QWidget):
    def __init__(self):
        super().__init__()
        self.initUI()
    def initUI(self):   
        grid = QGridLayout()  
        self.setLayout(grid)
        names = ['Cls', 'Bck', '', 'Close', 
        '7', '8', '9', '/',
        '4', '5', '6', '*',
        '1', '2', '3', '-',
        '0', '.', '=', '+',]
        positions = [(i, j) for i in range(5) for j in range(4)]

        for position, name in zip(positions, names):
         if name == '':
          continue
         button = QPushButton(name)
         grid.addWidget(button, *position)

        self.setWindowTitle('PyQt window')  
        self.show()
        
def main():    
    app = QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())         

if __name__ == '__main__':
    main()

When mmcv is imported to code and close the PyQT window, I have

free(): invalid pointer
Aborted (core dumped)

If import mmcv is removed, no error message. What is the issue?