Showing Run time Null Exception error

Question

TravelFlightInformationType_185853S _TravelFlightInfo = new TravelFlightInformationType_185853S();

_TravelFlightInfo.unitNumberDetail[0] = new NumberOfUnitDetailsTypeI1();

_TravelFlightInfo.unitNumberDetail[0].numberOfUnits = "3";

_TravelFlightInfo.unitNumberDetail[0].typeOfUnit = "PX";