Object reference not set to an instance of an object
This error happens when you try to use a property or call a method of an object that is null. More details:
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A simple use of Visual studio DEBUGGER can tell you the object because of which it is happening. Just look at the stack trace and put a debugger on that line. Check the objects of that line and see if any one is null and you are trying to use that objects property. Handle the same.
Since you have not shared the code, it's difficult to point possible places but the fix would be to handle the scenario such that whenever there is data for person in Db, fetch it and set it from the default selection list.
Steps:
1. Populate the State dropdown.
2. Set the selected value of dropdown based on the DB selection value for that user
3. Get the cities of the state based on the selection
4. populate the cities dropdown based on the cities fetched
5. Set the seletced value of dropdown based on the DB selected value for that user
Try!