Master -> Detail Relationship

If created several pages that are working perfectly using the Master-Detail Relationship tutorial video.

Here’s what stumped me: When clicking the row to initiate the display of the Detail data, it appears to use the variable value to display the result. However, when there is no result (no data), there is no indication when the data is available.

My question is, “What can I use as a trigger, and how can we use the error to alert the user that there is no data to report?”

Thanks for your help.