Fix crash when OnMapOpened delegate fires after SLevelEditorFlow destruction (#362)

The SLevelEditorFlow widget subscribes to FEditorDelegates::OnMapOpened using AddRaw, but never unsubscribes.
When a map is loaded and the widget has been destroyed, the delegate fires on a stale pointer causing an access violation.
Fix: Store the delegate handle and remove it in the destructor.

Author: EvanC4

Source/FlowEditor/Private/Utils/SLevelEditorFlow.cpp

@@ -14,7 +14,12 @@
 void SLevelEditorFlow::Construct(const FArguments& InArgs)
 {
 	CreateFlowWidget();
-	FEditorDelegates::OnMapOpened.AddRaw(this, &SLevelEditorFlow::OnMapOpened);
+	OnMapOpenedHandle = FEditorDelegates::OnMapOpened.AddRaw(this, &SLevelEditorFlow::OnMapOpened);
+}
+
+SLevelEditorFlow::~SLevelEditorFlow()
+{
+	FEditorDelegates::OnMapOpened.Remove(OnMapOpenedHandle);
 }
 
 void SLevelEditorFlow::OnMapOpened(const FString& Filename, bool bAsTemplate)

Source/FlowEditor/Public/Utils/SLevelEditorFlow.h

@@ -14,6 +14,8 @@ class FLOWEDITOR_API SLevelEditorFlow : public SCompoundWidget
 
 	void Construct(const FArguments& InArgs);
 
+	~SLevelEditorFlow();
+
 protected:
 	void OnMapOpened(const FString& Filename, bool bAsTemplate);
 	void CreateFlowWidget();
@@ -24,4 +26,5 @@ class FLOWEDITOR_API SLevelEditorFlow : public SCompoundWidget
 	static class UFlowComponent* FindFlowComponent();
 
 	FString FlowAssetPath;
+	FDelegateHandle OnMapOpenedHandle;
 };