The mod Vehicle deployer (1.0.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event vehicle-deployer::on_entity_destroyed (ID 159)
vehicle-deployer/control.lua:58: table index is nil
stack traceback:
vehicle-deployer/control.lua:58: in function <vehicle-deployer/control.lua:57>

I have come up with a solution but I'm not 100% sure that it won't have unintended side effects.
On line 57 of control.lua, the following function can be modified so that unit_number is checked prior to using the value as an array index.

script.on_event(ev.on_entity_destroyed, function(event)
global.deployers[event.unit_number] = nil
end)

to

script.on_event(ev.on_entity_destroyed, function(event)
if not event.unit_number == nil then
global.deployers[event.unit_number] = nil
end
end)

This should now be fixed with v1.0.4. Thanks for reporting this and sorry for taking so long to finally fix this small issue.

Does not seem to be fixed?

I just got this issue in my save too... without a single vehicle yet, when a furnace got replaced with a steel furnace by a bot. (using one of the quick start mods for early bots)

factorio-current.txt trace:

331.974 Error MainLoop.cpp:1382: Exception at tick 1898056: The mod Vehicle deployer (1.0.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event vehicle-deployer::on_entity_destroyed (ID 160)
vehicle-deployer/control.lua:63: table index is nil
stack traceback:
vehicle-deployer/control.lua:63: in function <vehicle-deployer/control.lua:57>

oh yeah thanks for reporting that. I accidentally made the condition the wrong way around.
should be fixed for real this time with v1.0.5