Attempt to fix script engine crash seen on SB5, SB6 and Plus SB.

[appurist]

• Avoid a race condition where the viewer might send (or a script might request) what results in a second _Replace(itemID, null) for a delete which is a duplicate. This can happen if the user deletes the TaskInventoryItem a second time before the viewer updates the Contents to remove it. There must be a script-based equivalent. This change just handle the itemID no longer being present in the items list. See the forum post reporting it.
• Also replaced the magic numbers with InventoryType constants.

[ddaeschler]

Race window is shortened, but isn't fixed. See my inline comment

[appurist]

I've committed another change that should take care of the rest. It's still possible to end up in the second half of that function on a duplicate request, but I don't think the effects of that problem warrant more changes, and we may be past the point of pragmatism on this one now.

[ddaeschler]

Looks good!