Uninstalling the upper receiver from the testing assembly crashes the game

[Hyperseeker]

Steps to reproduce:

1. Spawn ar15
2. Try to uninstall the upper receiver

Multiple tests have revealed that removing one or more children mods (i.e. mods which are attached to slots added by the upper receiver or its direct children) prevent the crash. Removing a sibling mod does not affect the outcome.

There appears to be an issue with trying to remove objects which aren't there. The following error has been observed in debug.log post-crash:

ERROR : src/visitable.cpp:502 [item visitable::remove_item(item&)] Tried removing item from object which did not contain it

Uninstalling other mods with children still attached has not produced crashes or errors. These uninstall gracefully, just like they do with the already-existing attachments.

[Hyperseeker]

After further testing with the Demo 1 pre-assembled AR-15s, it's become more clear that the issue is with pre-assembled guns: specifically, guns that have default mods defined in their entry.

This may have to do with how such guns are handled, or it could be an unfortunate consequence of having too many default mods on one gun, which is the default behavior of Gunsmith.

Transition to adding mods via spawn may resolve this issue.