You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.
Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.
Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"
I understand the difficulty explained in the comments to this answer. In more complex cases, there is a more radical technique.
Catch all exceptions of the very top stack frame of each thread. In other words, catch everything. When handle the exception (only one place per thread), get the exceptions stack trace: http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx
This is just a string. Write this string into the system log using
After running the application, read the log. The exception trace will show you all the path where the exception was propagated. Get to the code again, to find ends.
So far, it always worked out.