Glad to read that 
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About that bug – you probably threw a grenade, then picked in up, and then threw it again before it exploded. If you do that, the grenade’s explosion is triggered twice, but it gets destroyed in between, resulting in this error.
It simply shouldn’t be possible and it’s just a result of the way stuff is handled in the simplistic placeholder inventory-ish system included in the sample. In the same way, the grenades are just meant for presenting the explosion mechanics, as well as providing an example of how a simple grenade can be implemented, rather than being ready-to-use in a real project.