typo - registerListener needs to return the same type as unregister takes, also the design for the subscriber could be better however lets leave it currently

Author: AnastaZIuk

include/nbl/ext/ImGui/ImGui.h

@@ -57,7 +57,7 @@ class UI final : public core::IReferenceCounted
 		bool render(nbl::video::SIntendedSubmitInfo& info, const nbl::video::IGPUDescriptorSet* const descriptorSet);
 
 		//! registers lambda listener in which ImGUI calls should be recorded
-		std::optional<size_t> registerListener(std::function<void()> const& listener);
+		size_t registerListener(std::function<void()> const& listener);
 		std::optional<size_t> unregisterListener(size_t id);
 
 		//! sets ImGUI context, you are supposed to pass valid ImGuiContext* context

src/nbl/ext/ImGui/ImGui.cpp

@@ -1016,7 +1016,7 @@ namespace nbl::ext::imgui
 		return true;
 	}
 
-	std::optional<size_t> UI::registerListener(const std::function<void()>& listener)
+	size_t UI::registerListener(const std::function<void()>& listener)
 	{
 		assert(listener != nullptr);
 		m_subscribers.emplace_back(listener);