Hi,
as Albin said, the reason FirstOrEmpty would fail is when you dont get a table back from your REST call (so flow doesnt know what is the model for the empty record to return)
The solution that comes to my mind (actually came from this Forum Any FlowScript functions to check if a Table has a specific column? - #7 by PvanderHeyden so thank you @mew 
) is make sure you have the record using JSON encode and then manually provide an empty record in case it doesnt exist.
So something like this:
If it is possible that one of the tables within a table is missing even when you have the name =xx then you would need to check for “meas1” also.
(Although instr-ing often in a flow against a large string may have some performance drawbacks)
Hope this helps!
B R
Ivan
testb.flow (20.6 KB)