bluecode_apply crash when adding Blueprint class function node from UMG button event

[winyunq]

Summary

bluecode_apply can crash Unreal Editor while adding a Blueprint callable node from a UMG widget event. In this case the editor crashed inside UUmgBlueprintFunctionSubsystem::AddNode() with an access violation writing to 0x50.

Environment

• Project: LiteRTDemo
• Unreal Engine: 5.8
• UMG MCP target asset: /Game/AIWerewolf/WBP_AIWerewolfGame
• Active event graph target: StartGameButton.OnClicked
• MCP bridge port: 127.0.0.1:55557

Reproduction

1. Open a project using the UmgMcp plugin in UE 5.8.
2. Set the active UMG target to /Game/AIWerewolf/WBP_AIWerewolfGame.
3. Bind/focus a button event with bluecode_set_function("StartGameButton.OnClicked").
4. Search a Blueprint class function node successfully: query StartNewGame or Start New Game returns Start New Game, category |Class|BP AIWerewolf Director, node class K2Node_CallFunction.
5. Apply this bluecode with the returned action handle:

main()
  node("Start New Game")

Action handle used:

{"Start New Game":"bpact:5a885551b5f9da989c22124d1c396ef0"}

Crash

Unreal Editor crashed with:

Unhandled Exception: EXCEPTION_ACCESS_VIOLATION writing address 0x0000000000000050

UnrealEditor_UmgMcp!UUmgBlueprintFunctionSubsystem::AddNode() [D:\UE5Project\LiteRTDemo\Source\UmgMcp\Private\Blueprint\UmgBlueprintFunctionSubsystem.cpp:3867]
UnrealEditor_UmgMcp!UUmgBlueprintFunctionSubsystem::ApplyBluecode() [D:\UE5Project\LiteRTDemo\Source\UmgMcp\Private\Blueprint\UmgBlueprintFunctionSubsystem.cpp:6017]
UnrealEditor_UmgMcp!UUmgBlueprintFunctionSubsystem::ExecuteGraphAction() [D:\UE5Project\LiteRTDemo\Source\UmgMcp\Private\Blueprint\UmgBlueprintFunctionSubsystem.cpp:2899]
UnrealEditor_UmgMcp!UUmgBlueprintFunctionSubsystem::HandleBlueprintGraphAction() [D:\UE5Project\LiteRTDemo\Source\UmgMcp\Private\Blueprint\UmgBlueprintFunctionSubsystem.cpp:2746]
UnrealEditor_UmgMcp!UUmgMcpBridge::InternalExecuteCommand() [D:\UE5Project\LiteRTDemo\Source\UmgMcp\Private\Bridge\UmgMcpBridge.cpp:651]
UnrealEditor_UmgMcp!UUmgMcpBridge::ProcessNextQueuedCommand() [D:\UE5Project\LiteRTDemo\Source\UmgMcp\Private\Bridge\UmgMcpBridge.cpp:326]
UnrealEditor_Core
UnrealEditor_Core

Expected behavior

bluecode_apply should either create the call node, or return a structured failure if the selected function cannot be created without a Target/self pin context. It should not crash the editor.

Notes

Adjacent operations that were successful in the same asset before the crash:

• create_widget and set_widget_properties
• adding member variables with bluecode_apply_variables
• creating and wiring Download Gemma 4 E2B LiteRT-LM Model async action node
• creating variable set nodes for simple assignments

This may be related to missing/null Target pin or class context handling in AddNode() for non-static Blueprint class functions created from a UMG widget event graph.