I got the following error several times this afternoon, and took a screenshot of the stack. I was able to repeat it by building a requestor, assigning it an item, and putting in some drones. If I delete the requester before the first fuel arrived, I was able to repeatedly get this crash in "process_deliver_fuel", apparently as the fuel delivery looked to find its destination but got a stale handle.

I was planning to attach a .png with the stack trace, but can't find a way to do it in this interface. Can someone toss me a hint? It's probably obvious, but I'm not seeing it right now.

Since I can't do the screenshot yet, the error was:
Error while running event Transport_Drones::on_ai_command_completed (ID 113)
LuaEntity API call when LuaEntity was invalid.
stack traceback (hand copied):
transport_drone.lua:303: 'process_deliver_fuel'
transport_drone.lua:545: 'update'
transport_drone.lua:670: 'handler'

What version of the mod are you playing with?

after a crash, most of the time you should be able to drag the mouse over the text of the error notice and then ctrl-C copy it. if that doesn't work, you can look in the factorio directories for a file factorio-current.log or factorio-previous.log which near the end should list the same messages too.

if you want to use some markup for the text you write on the forum, search for "markup" on the internet, but there are lots of versions with small differences, eg for youtube or this mod portal. thus you also can simply go to another posting that uses the effect that you want and click the >"< symbol in the headline to "reply with citation" to see the complete text of the message with all markup.

but please remember to abort/discard that fake edit since you will not be able to delete any comment after you (accidentally) have submitted it. at most you can edit it to read "sorry, please ignore" instead of showing annoying text

Sorry for the delay - had a busy week, no time for forums.

The version was 0.6.0, per the stack. The image of the error should be here: https://imgur.com/a/abaCZqk

Next time I find something, I'll save off the log file for maximum context, and copy the text instead of screenshotting it. Frankly, it didn't even occur to me that the game would interact properly with the Clipboard - I'm so used to game displays being graphic-only that I didn't even try. :) Yet another good design decision, which I'll use in the future.

currently, that copy-trick doesn't work in all dialogs. eg it does work on mod crashes in a map, but it does not work while starting up the game and having mods that can't be loaded. it would be nice if this worked on all dialogs. but looking at the logfile should always show the reason for a crash.
let's wait and see how and where ctrl-c works when the update with all new gui dialogs is published ...