Fix invalid memory access on main window destruction

[jpcima]

Solves bad access into freed memory when the program is closed, found by address sanitizer.
On destruction of the comstack, it triggers a slot on the main window whose ui has been freed prior.

To prevent the problem, I ensure to destroy the ui last.
I put the variable in RAII and it's working because the members destroy in reverse order.

[rerrahkr]

Thanks for your contribution!