Null Reference Exception

Even if I set the gat, there is an error of null Reference exception

private PatientPlanDto selectedPatientPlan;

        public PatientPlanDto SelectedPatientPlan
        {
            get => selectedPatientPlan;
            set
            {
                selectedPatientPlan = value;
                RaisePropertyChanged();

            }
        }

if (SelectedPatientPlan.RemainingVisits>0)

RemainingVisit has value but does not accept. Why?