How to get rid of NullReferenceExcetion error

How to get rid of NullReferenceExcetion error

Hi I got this error NullReferenceException was unhandled by user code. The
error occurs in PropertyChanged(this, new
PropertyChangedEventArgs("AboveAircraft")); I tried if( this != null) and
it still got the error. How do I get rid of it. The code is like this:
public int AboveAircraft
{
get { return _above; }
set
{
if (SetProperty(ref _above, value, "AboveAircraft") &&
_updateModel)
{
if (Model.AltitudeBand == null)
{
Model.AltitudeBand = new AltitudeBand();
}
if (this != null && AboveAircraft != null)
{
PropertyChanged(this, new
PropertyChangedEventArgs("AboveAircraft"));
if (_above < _below)
{
BelowAircraft = _above;
}
}
Model.AltitudeBand .Above = new AltitudeBandLimit() { Unit
= AltitudeUnit.Foot, Value = _above };
}
}
}