Patricia - the problem with your screen was different.
The message you got was misleading. The problem was that your refresh grid action did not specify a grid. When I edited the Refresh Grid action on the screen the grid dropdown was blank. Once that was set to the Instruments by SN grid, and the screen got republished, then it worked like it should.
I have put a ticket in so that on future versions we'll change the message you get in that situation so that it tells you that no grid was assigned, rather than that ambiguous message.
Paul