fix(blueprint): prevent editor crash creating ConstructObjectFromClass nodes (SpawnActorFromClass)

[Fl0p]

Summary

Fixes #499 — manage_blueprint → create_node hard-crashes the editor for
K2Node_SpawnActorFromClass (and the rest of the UK2Node_ConstructObjectFromClass
family: ConstructObjectFromClass, CreateWidget, …).

Root cause (corrected vs. the issue)

The issue blamed AllocateDefaultPins() + FindPinChecked and suggested FGraphNodeCreator.
That attribution is off by one call — and FGraphNodeCreator would not have fixed it (its
Finalize() runs the same fatal order). The real culprit is PostPlacedNewNode():

void UK2Node_SpawnActorFromClass::PostPlacedNewNode() {
    UEdGraphPin* const ScaleMethodPin = GetScaleMethodPin(); // FindPinChecked(TransformScaleMethod)
    ScaleMethodPin->DefaultValue = ...;                      // check(Result) fires — pin doesn't exist yet
}

CreateDynamicNode calls PostPlacedNewNode() while Pins.Num() == 0, so the checked pin
accessor asserts and the editor dies. The engine's own palette spawn survives because the cached
template node already carries pins by the time PostPlacedNewNode runs on the placed copy.

Fix

Add TryCreateConstructObjectNode() (in …SpecialNodes.cpp), dispatched from CreateDynamicNode
right after the existing TryCreateEnhancedInputNode check. It:

• gates on NodeClass->IsChildOf(UK2Node_ConstructObjectFromClass::StaticClass()) — covers the
  whole family in one place;
• allocates default pins before PostPlacedNewNode() (the core fix), mirroring the
  production-proven TryCreateEnhancedInputNode path (Modify, ForceVisualizationCacheClear,
  NotifyGraphChanged, throttled save);
• optionally seeds a class from a spawnClass / actorClass / class payload field
  (ClassPin->DefaultObject + ReconstructNode()) so the expose-on-spawn pins are generated — a
  classless node is still valid and no longer crashes;
• leaves the generic NewObject path and its FunctionResult double-pin guard untouched.

Files

• …/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersSpecialNodes.cpp — new helper + #include "K2Node_ConstructObjectFromClass.h"
• …/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersPrivate.h — declaration
• …/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersNodeCreation.cpp — wire into dispatch

No .Build.cs change (BlueprintGraph already a private dependency). No TS-bridge change required;
forwarding the optional class field through the bridge schema is a possible follow-up.

Testing

• create_node nodeType=K2Node_SpawnActorFromClass → node created, editor stays alive (was 2/2 crash on UE 5.6).
• With a spawnClass payload → expose-on-spawn pins appear; connect_pins works; blueprint compiles.
• No regression: K2Node_FunctionResult still yields a single execute pin; simple nodes (get/set, math) unaffected.
• Sibling family sanity: ConstructObjectFromClass, CreateWidget.

Root cause verified against UE5 source and a working reference plugin (allocates pins first, skips
PostPlacedNewNode entirely); the order-swap is class-scoped and low-risk. I haven't been able to
run a full UE 5.6 editor build on my end — happy to iterate if your CI/build surfaces anything.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 9d42c807-a0dc-4b0c-a1b7-7d4f93c43cf5

📥 Commits

Reviewing files that changed from the base of the PR and between eb6f48e and 5872c95.

📒 Files selected for processing (1)

• plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersSpecialNodes.cpp

🚧 Files skipped from review as they are similar to previous changes (1)

• plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersSpecialNodes.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved Blueprint node creation reliability in the editor.
  • Fixed hard-crash issues when creating construct-object-style nodes that require pins to be initialized during placement.
  • These nodes now spawn safely with pins and class settings correctly set, including proper expose-on-spawn pin generation.
  • Graph updates and change notifications now occur cleanly after node creation.

Walkthrough

The PR adds an editor-only helper for creating UK2Node_ConstructObjectFromClass-family nodes with pins allocated before PostPlacedNewNode(), and updates CreateDynamicNode to try that helper before the existing generic node creation path.

Changes

Construct-object node creation

Layer / File(s)	Summary
Specialized helper and pin finalization plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersPrivate.h, plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersSpecialNodes.cpp	TryCreateConstructObjectNode is declared and implemented for UK2Node_ConstructObjectFromClass subclasses, allocates pins before PostPlacedNewNode(), applies optional class payload values, and returns a JSON success response.
Generic dispatch to the specialized path plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersNodeCreation.cpp	CreateDynamicNode calls TryCreateConstructObjectNode first and returns early when it succeeds before continuing into the generic NewObject creation flow.

Sequence Diagram(s)

sequenceDiagram
  participant CreateDynamicNode
  participant TryCreateConstructObjectNode
  participant TargetGraph
  participant Blueprint

  CreateDynamicNode->>TryCreateConstructObjectNode: Try construct-object node handling
  TryCreateConstructObjectNode->>TargetGraph: AddNode(), CreateNewGuid(), AllocateDefaultPins(), PostPlacedNewNode()
  TryCreateConstructObjectNode->>Blueprint: MarkBlueprintAsModified(), SaveLoadedAssetThrottled()
  TryCreateConstructObjectNode-->>CreateDynamicNode: Success response with nodeId, nodeName, nodeClass

Loading

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• ChiR24/Unreal_mcp#460: Both PRs change CreateDynamicNode with node-family-specific early handling to avoid editor crashes.
• ChiR24/Unreal_mcp#471: Both PRs adjust BlueprintGraph node creation around pin allocation and PostPlacedNewNode() timing.

Suggested labels

bug

Suggested reviewers

• ChiR24

Poem

A rabbit hopped through pins so neat,
and found the class before it leapt.
No crash! The graph stayed bright and clear,
with pin-sprouts blooming bunny-cheer.
Thump-thump—success in fluffy gear. 🐰

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main fix: preventing editor crashes when creating ConstructObjectFromClass-family nodes.
Description check	✅ Passed	The description includes a solid summary, root cause, fix, files changed, related issue, and testing notes; only template checklist sections are missing.
Linked Issues check	✅ Passed	The changes address #499 by adding a safer creation path that allocates pins before PostPlacedNewNode and supports class seeding/reconstruction.
Out of Scope Changes check	✅ Passed	The added helper, declaration, include, and dispatch change are all directly tied to the crash fix and related node creation flow.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In
`@plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersSpecialNodes.cpp`:
- Around line 230-277: The special node creation path in
McpAutomationBridge_BlueprintGraphHandlersSpecialNodes should reject invalid
spawn class payloads instead of silently succeeding with an untyped node. In the
node setup flow around NewNode, resolve the requested class from
spawnClass/actorClass/class before mutating the graph, and if ResolveUClass()
fails, return an error via Context.SendError with CLASS_NOT_FOUND rather than
calling AddNode() and continuing. Keep the existing
AllocateDefaultPins/PostPlacedNewNode/ReconstructNode logic in the successful
path, but only after the class is validated and applied.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 753dd497-66cf-49d9-954f-6216f2ff2dab

📥 Commits

Reviewing files that changed from the base of the PR and between d49b158 and eb6f48e.

📒 Files selected for processing (3)

• plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersNodeCreation.cpp
• plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersPrivate.h
• plugins/McpAutomationBridge/Source/McpAutomationBridge/Private/Domains/BlueprintGraph/McpAutomationBridge_BlueprintGraphHandlersSpecialNodes.cpp

[ChiR24]

Hey! Nice catch on the root cause — allocating default pins before PostPlacedNewNode() is exactly right.

Two things before we merge though:

1. The new TryCreateConstructObjectNode check currently sits above the CreateWidget handler. Since CreateWidget is a subclass of ConstructObjectFromClass, it gets swallowed by the new path and silently drops the targetClass — so typed widget nodes break. Moving the call below the CreateWidget block fixes it.
2. The new path reads spawnClass/class and uses ResolveUClass, but the rest of create_node uses targetClass + ResolveTargetClassFromString. That means /Game/... Blueprint paths fail with CLASS_NOT_FOUND for SpawnActor/ConstructObject too.