I made below MDI app thanks to AIs.
enter image description here

Currently, I realize that there’s a bug in my code which I am not sure.
Because same app was initially developed with Qt5 and at that time there’s no issue on it.
It started with the change from Qt5 to PySide6.
From below I’d like to explain even very simple.

1. in the menubar, select file and then execute “Infos”
2. in the menubar, select input and then execute “Inputs”
3. close “Input Master Infos” using “X” on the title menubar
4. close “Input Transaction List” using “X” on the title menubar

enter image description here
as above first cycle ended, now go to next step same as above

5. in the menubar, select file and then execute “Infos”

6. in the menubar, select input and then execute “Inputs”

7. close “Input Master Infos” using “X” on the title menubar

8. close “Input Transaction List” using “X” on the title menubar
   as above second cycle ended, now go to next step

9. in the menubar, select file and then execute “Infos”

10. in the menubar, select input and then execute “Inputs”

11. while closing “Input Master Infos” using “X” on the title menubar, suddenly main window closed without any logs.

Above is the pattern I noticed but not settled down.
I hope to know the bug and fix it but I have not enough knowledge to solve it.
Please help me to make it.

This is my first time posting to stackoverflow so it is hard to share my code, so I added the code link as below.

https://github.com/hotbear88/Share