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: here
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.