Object reference not set to an instance of an object, Interface

Object reference not set to an instance of an object, Interface

I know that this is one of the most encountered error but I am really
struggling to get around it. i have a property on my Controller: private
readonly ISelectListFactory _selectFactory; and a method that called to
populate the viewbag private void PopulateLists() { var continent =
_selectFactory.GetItems(); } and the interface
public interface ISelectListFactory
{
IEnumerable<SelectListItem> GetItems();
}
and in the controller constructor I have the following:
public LocationController(ISelectListFactory selectFactory)
{
_selectFactory = selectFactory;
}
but I am getting this error Object reference not set to an instance of an
object and not sure how to overcome it.
Regards