Require/Check for Entry in Prior Field

Require/Check for Entry in Prior Field

I have a reservation form with a property field (dropdown) and a check in date field.  The user enters the property and then the check in date and the system looks up the available rates.  The issue occurs if the user skips the property field and then enters the check in date, the system then crashes with an error (as it is trying to look up a null value).

Is there a way for me to check if the prior field (property) has an entry upon entry of the second field (check in date)?  I am thinking this should be easy with an if statement on user input. Then if there is NO entry in this field, I would like to pop an alert to the user and then force them back to that field (property field).  Would that be possible?  If so how?