Fixed hot-reloading issues. Hot-reloading may be useful when developing this plug-in but it doesn't play well with free functions used in ImGui interface (limitation of the system I guess). Switched to a custom GImGui which after hot-reload can be redirected to a new module. This fixes a crash caused by GImGui in obsolete module pointing to released resources and guarantees that ImGui code remains functional.

Source/ImGui/Private/ImGuiImplementation.cpp

@@ -15,6 +15,15 @@
 #define IMGUI_DISABLE_WIN32_DEFAULT_IME_FUNCTIONS
 #endif // PLATFORM_XBOXONE
 
+#if WITH_EDITOR
+// Global ImGui context pointer.
+ImGuiContext* GImGuiContextPtr = nullptr;
+// Handle to the global ImGui context pointer.
+ImGuiContext** GImGuiContextPtrHandle = &GImGuiContextPtr;
+// Get the global ImGui context pointer (GImGui) indirectly to allow redirections in obsolete modules.
+#define GImGui (*GImGuiContextPtrHandle)
+#endif // WITH_EDITOR
+
 #if PLATFORM_WINDOWS
 #include <AllowWindowsPlatformTypes.h>
 #endif // PLATFORM_WINDOWS
@@ -51,4 +60,16 @@ namespace ImGuiImplementation
 			return false;
 		}
 	}
+
+#if WITH_EDITOR
+	ImGuiContext** GetImGuiContextHandle()
+	{
+		return GImGuiContextPtrHandle;
+	}
+
+	void SetImGuiContextHandle(ImGuiContext** Handle)
+	{
+		GImGuiContextPtrHandle = Handle;
+	}
+#endif // WITH_EDITOR
 }

Source/ImGui/Private/ImGuiImplementation.h

@@ -10,4 +10,12 @@ namespace ImGuiImplementation
 {
 	// Get specific cursor data.
 	bool GetCursorData(ImGuiMouseCursor CursorType, FVector2D& OutSize, FVector2D& OutUVMin, FVector2D& OutUVMax, FVector2D& OutOutlineUVMin, FVector2D& OutOutlineUVMax);
+
+#if WITH_EDITOR
+	// Get the handle to the ImGui Context pointer.
+	ImGuiContext** GetImGuiContextHandle();
+
+	// Set the ImGui Context pointer handle.
+	void SetImGuiContextHandle(ImGuiContext** Handle);
+#endif // WITH_EDITOR
 }

Source/ImGui/Private/ImGuiModule.cpp

@@ -7,6 +7,7 @@
 #include "Utilities/WorldContextIndex.h"
 
 #if WITH_EDITOR
+#include "ImGuiImplementation.h"
 #include "Editor/ImGuiEditor.h"
 #endif
 
@@ -138,8 +139,33 @@ void FImGuiModule::ShutdownModule()
 	checkf(ImGuiModuleManager, TEXT("Null ImGui Module Manager. Module manager instance should be deleted during module shutdown."));
 	delete ImGuiModuleManager;
 	ImGuiModuleManager = nullptr;
+
+#if WITH_EDITOR
+	// When shutting down we leave the global ImGui context pointer and handle pointing to resources that are already
+	// deleted. This can cause troubles after hot-reload when code in other modules calls ImGui interface functions
+	// which are statically bound to the obsolete module. To keep ImGui code functional we can redirect context handle
+	// to point to the new module.
+	FModuleManager::Get().OnModulesChanged().AddLambda([this](FName Name, EModuleChangeReason Reason)
+	{
+		if (Reason == EModuleChangeReason::ModuleLoaded && Name == "ImGui")
+		{
+			FImGuiModule& LoadedModule = FImGuiModule::Get();
+			if (&LoadedModule != this)
+			{
+				ImGuiImplementation::SetImGuiContextHandle(LoadedModule.GetImGuiContextHandle());
+			}
+		}
+	});
+#endif // WITH_EDITOR
 }
 
+#if WITH_EDITOR
+ImGuiContext** FImGuiModule::GetImGuiContextHandle()
+{
+	return ImGuiImplementation::GetImGuiContextHandle();
+}
+#endif
+
 bool FImGuiModule::IsInputMode() const
 {
 	return CVars::InputEnabled.GetValueOnAnyThread() > 0;

Source/ImGui/Public/ImGuiModule.h

@@ -110,4 +110,10 @@ public:
 	/** IModuleInterface implementation */
 	virtual void StartupModule() override;
 	virtual void ShutdownModule() override;
+
+	private:
+
+#if WITH_EDITOR
+	virtual struct ImGuiContext** GetImGuiContextHandle();
+#endif
 };