this happened when i deconstructed the rocket at the start

148.893 Error MainLoop.cpp:1404: Exception at tick 4367: The mod Zerwiport's Zoom (1.7.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event zoom-mod::zoom-mod_update-view-on-move-left (ID 336)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__zoom-mod/model/ClosedGuiEvent.lua:30: in function 'locomotive'
zoom-mod/model/ClosedGuiEvent.lua:20: in function 'mayChangePosition'
zoom-mod/model/Interface.lua:113: in function 'updateZoom'
zoom-mod/model/Interface.lua:107: in function 'updatePosition'
zoom-mod/model/Interface.lua:91: in function 'update'
zoom-mod/control.lua:37: in function <zoom-mod/control.lua:35>

this should fix it :)

if self._on_gui_closed.gui_type == defines.gui_type.entity and self._on_gui_closed.entity.valid and self._on_gui_closed.entity.name == "locomotive" then

Thanks for reporting. I'm going to get rid of accessing of an entity in the next ticks otherwise the functionality won't work sometimes.

valid :: booleanRead
Is this object valid? This Lua object holds a reference to an object within the game engine. It is possible that the game-engine object is removed whilst a mod still holds the corresponding Lua object. If that happens, the object becomes invalid, i.e. this attribute will be false. Mods are advised to check for object validity if any change to the game state might have occurred between the creation of the Lua object and its access.

I hope that not using it in the following ticks will be sufficient. If not, I will add this condition.

You may play with version 1.7.1

Because the documentation is not precise enough when invalidation of an entity may occur I've just added precautious check. Along with the changes from version 1.7.1 it should reduce occurrences of not updating position and zoom after opening the map on a locomotive and eliminate crashes.

thanks, things seem fine now :)

that said, i don't know if the rocket wrecks at the start of a game are indeed locomotives, and those caused the crash

It was destroyed after you closed the gui and in the following tick the gui's entity was invalid.