select command in gridview after publish return "error Object reference not set to an instance of an object"

Hi everybody,

I have a GridView with Select command. In localhost everything works well. When pubblishing the package, clicking on "select" Grid command I get the error below.
Question: wich object is not set? how to know where the error was originate? 
Thanks in advance
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Server Error in '/ServiceAttestatoCorsi' Application. Object reference not set to an instance of an object. Description: An unhandled exception occu ...