Object reference not set to an instance of an object.

Hi
AS I said I am new in asp.net

when i put data in gridview footer Insert this error comes
May i know the reason and how to solve this

Object reference not set to an instance of an object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.NullReferenceException: Object reference not set to an instance of an object. Source Error:
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