[nostalgia] Add NewMenu for creating new files

src/nostalgia/core/assetmanager.hpp

@@ -41,7 +41,7 @@ class AssetContainer {
 		AssetContainer& operator=(AssetContainer&&) = delete;
 
 		constexpr T *get() noexcept {
-			 return &m_obj;
+			return &m_obj;
 		}
 
 		constexpr const T *get() const noexcept {
@@ -92,7 +92,10 @@ class AssetRef: public ox::SignalHandler {
 		}
 
 		constexpr const T *get() const noexcept {
-			return m_ctr->get();
+			if (m_ctr) {
+				return m_ctr->get();
+			}
+			return nullptr;
 		}
 
 		constexpr const T &operator*() const & noexcept {