See Design Pattern: Associated Items for a full discussion. But, as rossko57 indicates, the code you have above is trying to use the name of triggeringItem so you already have access to that Item. When later when you postUpdate using itemNameStr, you can use the postUpdate Action.
postUpdate(itemNameStr, avr)
This is one of the few cases where use of the Action is OK.
In those cases where you need the state of an Item where you only have that Item’s name, Olti’s approach is recommended.
Both of these are covered in the Associated Items DP.