I have a 0.17.31 save with entirely too many mods (screenshot: https://i.imgur.com/fJJjCFR.png), and I'm getting this error when I click the UI button in the upper-left corner:

The mod How Far Has it Been? caused a non-recoverable error.
Please report this error to the mod author.

Error while running event howfardiditgo::on_gui_click (ID 1)
__howfardiditgo__/control.lua:243: attempt to index field '?' (a nil value)
stack traceback:
    __howfardiditgo__/control.lua:243: in function <__howfardiditgo__/control.lua:188>

If my better half hadn't just finished dinner I'd check out that control.lua file, but hopefully the report is helpful. Thanks for the sweet mod!

Edit: Markdown markup

Thanks for this, just realised a stupid oversight on my part. Just to confirm, you have a train somewhere that doesn't have a locomotive? :)

This should be fixed now in HFHIB 0.17.3. Thanks!

... Make that 0.17.4. :) Locomotive-less trains don't have a schedule to filter by, so while 0.17.3 made the HFHIB menu openable without crashing, it was still crashing if you tried to use the filter... Fixed now!

Ahh that makes sense...

Just to confirm, you have a train somewhere that doesn't have a locomotive? :)

Oh gosh, many- It was tedious to rebuild the schedules for my defense trains when they got eaten, so I started keeping a memory bank next to my bigger stations : D. https://i.imgur.com/xxfSSwI.png

It'll be fun to look at my list tomorrow!

Edit: Er, I misread that, I have trains without cars in that screenshot, but not a train without a locomotive! Heh, I did just install Rampant though, so I probably have an orphan getting gnawed on somewhere...