Initializing a Shift Register simply means to connect a value to the left Shift Register terminal from outside the loop. If a Shift Register is not initialized, it will store the latest value between executions of the VI. In your case, if you get an error, the While Loop will finnish and your VI will stop. Since the Shift Register is not initioalized, it will remember the error the next time the VI is executed, and therefore you will keep getting the error until you restart LabVIEW.
I have attached you example with a few changes that passes the error cluster and the VISA reference.
- Philip Courtois, Thinkbot Solutions