Symptoms

'NullReferenceException' when clicking the 'Save' toolbar button on a list view. 'SmartObject property PROPERTYNAME is a required property for selected method Create. Value must be set.
 

Diagnoses

A child list view on a parent form was trying to save the parent's ID with the child's list data. The parent ID did not actually exist yet, because this was a 'New' form and the parent had no real ID yet, it had not yet been created.
 

Resolution

Move the 'Create' method on the list view to a button where the whole page's data gets saved (i.e. a 'Submit' button) but after the parent form's 'Create' method. This way, the return ID from the creation of the parent form can be passed into the 'Create' method of the child view, and they will be properly associated.