Added null check to prevent error on scene load

Error Details:
NullReferenceException: Object reference not set to an instance of an object
at TarsierSpaceTech.TSTProgressTracker.OnLoad (.ConfigNode node) [0x00000] in <filename unknown>:0 
at ScenarioModule.Load (.ConfigNode node) [0x00000] in <filename unknown>:0 
at ScenarioRunner.AddModule (.ConfigNode node) [0x00000] in <filename unknown>:0 
at ProtoScenarioModule.Load (.ScenarioRunner host) [0x00000] in <filename unknown>:0 
at ScenarioRunner+.MoveNext () [0x00000] in <filename unknown>:0