It's not clear what is triggering the popup but, assuming it is a Save button or similar I can think of two solutions.
- Make the popup modal. That prevents access to the background.
- Don't enable the Save button until all the required fields have been filled out. Add an indicator beside each field (an icon would do nicely) to show that the field isn't ready and add a tooltip to explain what is required.
See Trouble with 'if' statement, button event - #5 by Transistor for more on idea 2.