cannot convert from 'System.Guid?' to 'System.Guid'

Obj GetValue(Guid yourID)
{
return FetchObject(yourID)
}
Void main()
{
Guid? passvalue;
Obj test = GetValue(passvalue.Value);
}

Guid yourGuid= Guid.NewGuid()

Guid defaultId = Guid.NewGuid();
 if (customerRow.GuardianState.Length > 2) {
 Guid StateId = record.StateId ?? defaultId;}