Hi guys,
I agree with voodoo_ that the issue is that you are calling DAQmx Read when there are no samples available. In this case, DAQmx Read will return a null array (since you have not specified the number of samples to read). Since it looks like you are only interested in a software timed update, I would recommend modifying your code to the following:
While inserting a wait into the while loop would likely also remedy the problem, I don't see the need for a continuous acquisition in this example. If you do need continuous acquisition, you can connect the Number of Samples to Read Per Channel input of DAQmx Read which will delay the loop according to how many samples you want to acquire per iteration.
-John
John Passiak