Credit system BP: fix crashes when unloading then reloading UE levels

IsValid was a first attempt at fixing these crashes which were witnessed
only in Shipping mode, ie. with no way of exactly knowing what was going
on. But IsValid itself was apparently using CesiumCreditSystemBP after
its deletion, so I ended up adding the object to the GC's root set.

Author: GhisBntly

Diff for: Source/CesiumRuntime/Private/CesiumCreditSystem.cpp

@@ -93,10 +93,15 @@ ACesiumCreditSystem::GetDefaultCreditSystem(const UObject* WorldContextObject) {
   // loader object that retrieves the blueprint class in its constructor. We can
   // destroy the loader immediately once it's done since it will have already
   // set CesiumCreditSystemBP.
-  if (!CesiumCreditSystemBP) {
+  if (!IsValid(CesiumCreditSystemBP)) {
     UCesiumCreditSystemBPLoader* bpLoader =
         NewObject<UCesiumCreditSystemBPLoader>();
     CesiumCreditSystemBP = bpLoader->CesiumCreditSystemBP.LoadSynchronous();
+    // Prevent the GC from destroying the object when unloading a level, as it will be reused
+    // if one subsequently loads another level. This kind of static object should probably be
+    // replaced by a "subsystem"
+    // (https://dev.epicgames.com/documentation/en-us/unreal-engine/programming-subsystems-in-unreal-engine)
+    CesiumCreditSystemBP->AddToRoot();
     bpLoader->ConditionalBeginDestroy();
   }