Hi,
if you can isolate this, it will help.
make sure you have just one fsm doing just that, and then debug it as Kurt said, it's likely empty at the time of execution and then got filled up.
turn on debugflow for example, and step back, you'll see the values at the time of execution.
that, or another fsm or script is setting that image sprite back to null right after your fsm.
I tested here and it works fine, I did create a fsmVar or type sprite and it worked ( the fsm sprite variable is exactly that, but beautified by the playmaker editor).
Bye,
Jean