Crash when mining/deconstructing nixie tubes

```
Error while running the event handler: __nixie-tubes__/control.lua:202: LuaEntity API call when LuaEntity was invalid.
```

I haven't been able to remove any of the tubes I've placed. Same happens when mining, deconstructing with bots, shooting, or using .destroy() on the entity from the console.

I'm running Narc's unofficial nixie tubes 0.1.8 on factorio 0.12.33.
