While i am saving my coaching report it is throwing an arror "System.NullPointerException: Argument cannot be null".Please help me out why iam getting this error message?

Iam having 3 Fields in one object

1.Picklist(Yes or No)
2.Checkbox(Marked)
3.Checkbox(Unmarked)

When ever an attachment is attached the third field will be marked if Picklist Value is YES & 2nd Checkbox is marked.