[NetImGui] Crash when running without a renderer

[KABoissonneault]

In changelist 0e4fa23, the LoadTextures in ImGuiModuleManager.cpp was moved from FImGuiModuleManager::AddWidgetToViewport to FImGuiModuleManager::OnContextProxyCreated. Since the proxy seems to be always created on first Tick even without a renderer, this crashes with the following callstack.

 	[Inline Frame] UnrealEditor-SlateCore.dll!FSlateBrush::CanRenderResourceObject(UObject *) Line 137	C++
>	UnrealEditor-SlateCore.dll!FSlateBrush::SetResourceObject(UObject * InResourceObject) Line 148	C++
 	UnrealEditor-ImGui.dll!FTextureManager::FTextureEntry::FTextureEntry(const FName & InName, UTexture2D * InTexture, bool bAddToRoot) Line 140	C++
 	UnrealEditor-ImGui.dll!FTextureManager::CreateTextureInternal(const FName & Name, int Width, int Height, unsigned int SrcBpp, unsigned char * SrcData, TFunction<void __cdecl(unsigned char *)> SrcDataCleanup) Line 68	C++
 	UnrealEditor-ImGui.dll!FTextureManager::CreatePlainTextureInternal(const FName & Name, int Width, int Height, const FColor & Color) Line 100	C++
 	[Inline Frame] UnrealEditor-ImGui.dll!FTextureManager::InitializeErrorTexture(const FColor &) Line 13	C++
 	[Inline Frame] UnrealEditor-ImGui.dll!FImGuiModuleManager::LoadTextures() Line 85	C++
 	UnrealEditor-ImGui.dll!FImGuiModuleManager::OnContextProxyCreated(int ContextIndex, FImGuiContextProxy & ContextProxy) Line 232	C++

On this line

if (FSlateApplicationBase::IsInitialized())
{
	return FSlateApplicationBase::Get().GetRenderer()->CanRenderResource(*InResourceObject);
}

With the error:

Exception thrown: read access violation.
FSlateApplicationBase::CurrentBaseApplication.Object->Renderer.Object was nullptr.

To reproduce, I opened the editor with -run=GenerateSoundBanks.