When you're in Spectator mode, this message appears in chat whenever you hover over an entity:

scripts/reviver.lua: 136: attempt to index field 'cursor_stack' (a nil value)

Apparently LuaPlayer::cursor_stack is nil ONLY in Spectator mode, which has caught me a few times as well.

While I'm not entirely sure how to enter spectator mode (is this a multiplayer-only feature?), I added a check to ensure cursor_stack exists (not nil) before attempting to check if cursor_stack.valid_for_read, should hopefully prevent that message from occurring. Implemented in v1.0.13

It seems like the issue is fixed.

You can enter Spectator mode with this command:

/c game.player.set_controller{type=defines.controllers.spectator}

then re-enter your character later with it selected under the cursor:

/c game.player.set_controller{type=defines.controllers.character, character=game.player.selected}

I think it's a very obscure feature used for scenarios and other mods.

Glad to hear that the issue is resolved, and good to know, thanks!