in the past few days, I had a lot of headaches fighting with the interaction among UI and slots, but I think that the result was worth the time I have spent on in.
with previous code, there were a lot of cases where the internal UI was not handling properly devices that creates further image or slot devices, because when the menu were first created there were not many devices triggering subdevices as we have now.
to make a few examples, in 0.157 if you launch sms and then try to mount the multitap option in Slot Options, after Reset the multitap is stuck using 4 joypads and the corresponding options cannot be configured from UI
again, in 0.157 if you launch fds and then you mount the Game Genie from the UI, the passthru slot does not acknowledge any image you attempt to mount from the File Manager (you can mount it, but the system reboots and the second cart is gone)
finally, in 0.157 if you mount c64 with no carts inserted and then add the cbmieee cart from the File Manager, you can get both previous bugs combined: at reboot, neither the -cart2 device nor the slot sub-options would work as they should
with current git tree all the above behaviors have been rectified, and the sub-options can be finally configured as one would expect