I wasn’t able to reproduce the crash locally on a fresh UE5.5 project using -server or PIE with a dedicated server.
Looking at the source code, the crash appears to occur in this section:
UWidgetTree* Tree = NamedSlotClass->GetWidgetTreeArchetype();
TArray<FName> SlotNames;
Tree->GetSlotNames(SlotNames);
If Tree is nullptr here then it would explain the access violation. However, in my tests, this method is called when compiling the Editor Utility Widget, and I wasn’t able to get a nullptr from GetWidgetTreeArchetype().
Could you provide a minimal reproducible project where the crash happens or at least more detailed repro steps?