Crash: check(Event.IsValid()) in StateTreeEvents.h:119 — CaptureNewStateEvents treats an unchanged frame's states as newly entered

General Issues and Bug Reporting (Unreal Engine)
1

Summary

FStateTreeExecutionContext::CaptureNewStateEvents (StateTreeExecutionContext.cpp, ~line 2503) initializes UniqueStates to all of NewFrame.ActiveStates and only narrows it when the StateID comparison loop finds a divergence. For a frame whose active states are fully identical to the previous frame (an untouched parent frame while a transition re-selects inside a linked-asset frame below it), the loop never breaks, so every state is treated as newly entered.

Because state selection skips re-walking the common prefix (~line 5182), the identical frame’s per-depth entries in FramesStateSelectionEvents are never written and are empty. If that frame contains two or more active states with “Event Required to Enter”: the first empty entry takes the “steal” branch and adds an invalid FStateTreeSharedEvent to EventsInUse (~2546); the second empty entry then matches it (two null TSharedPtrs compare equal), takes the copy branch, and EventToCapture (~2541) trips check(Event.IsValid()) in FStateTreeSharedEvent::operator (StateTreeEvents.h:119).

Side effect even with a single required-event state: the steal branch overwrites the state’s stored captured event with the empty one, which breaks event-payload bindings and fires the ensure(SharedEvent.IsValid()) in StateTreeInstanceData.cpp:284 on the next read.

What Type of Bug are you experiencing?

AI

Steps to Reproduce

  1. Create a StateTree sub-asset ST_Sub (Component schema) with two sibling child states A and B; give A a Delay task and an On State Completed → B transition.
  2. Create a main StateTree: under Root, a state P1 with Event Required to Enter (tag Test.Event, any payload struct), and its child P2 as a Linked Asset state pointing at ST_Sub, also with Event Required
    to Enter Test.Event (both with “Consume Event On Select” off).
  3. Add a transition into P1 on event Test.Event, run the tree on a StateTreeComponent, send the event.
  4. When A’s delay completes and the internal A → B transition fires inside the linked frame, the assert hits.

Expected Result

I can transition states without a crash

Observed Result

Unreal Crashes

Affects Versions

5.6

Platform(s)

Windows