EntityCommands::insert panics when the entity is despawned, which is a footgun

[SludgePhD]

EntityCommands::insert will panic if the entity it is adding a bundle to is removed by any other system in the same tick, unless there's an apply_deferred system that runs between those systems.

This is often very difficult to notice, since the insertion might conflict with an entirely unrelated system that just so happens to also run in the Update schedule. In my particular case, I believe I have accidentally made a preexisting race condition like this much more likely by loosening some unnecessary ordering constraints between some completely unrelated systems.

I believe that EntityCommands::insert could be made much less difficult to use correctly if its behavior with a non-existent entity would instead be to simply do nothing, rather than to panic. This would also match the behavior of EntityCommands::remove and remove_resource, which do nothing when the thing they remove doesn't exist, or when their entity doesn't exist. Thoughts?

[SludgePhD]

Ah, this is a duplicate of #7118.