Item group or subgroup.
Is this object valid? This Lua object holds a reference to an object within
the game engine. It is possible that the game-engine object is removed whilst
a mod still holds the corresponding Lua object. If that happens, the object
becomes invalid, i.e. this attribute will be false. Mods are advised to
check for object validity if any change to the game state might have occurred
between the creation of the Lua object and its access.
The parent group if any; nil if none.
Subgroups of this group.