I have a C++ UUserWidget class, and from that I derived a blueprint class. In the C++ class, I created a function and added the BlueprintNativeEvent Macro to it in order to be able to call the event from both c++ and blueprints and be able to add code from both. The event is being called from both…

You’re calling RefreshAll from the constructor. The widget assigned to MainTitle won’t have been created yet. You shouldn’t make function calls like that from constructors in UE, you should only set default values. You’d want to call this from NativeConstruct.