AnySceneImporter: fix meshAttribute[For]Name() without a file opened.

doc/changelog.dox

@@ -592,6 +592,10 @@ See also:
     [mosra/magnum#523](https://github.com/mosra/magnum/issues/523).
 -   `UseEmscripten` is no longer implicitly included on Emscripten as it's not
     needed for anything (see [mosra/magnum#490](https://github.com/mosra/magnum/issues/490))
+-   Fixed an assertion in @relativeref{Trade,AnySceneImporter} when
+    @relativeref{Trade::AbstractImporter,meshAttributeName()} or
+    @relativeref{Trade::AbstractImporter,meshAttributeForName()} was called
+    without a file opened
 -   @ref Trade::ObjImporter "ObjImporter" uses exceptions internally and needs
     an explicit exception-enabling flag when built with Emscripten 1.39.0 and
     newer

src/MagnumPlugins/AnySceneImporter/AnySceneImporter.cpp

@@ -231,8 +231,16 @@ Int AnySceneImporter::doMeshForName(const Containers::StringView name) { return
 Containers::String AnySceneImporter::doMeshName(const UnsignedInt id) { return _in->meshName(id); }
 Containers::Optional<MeshData> AnySceneImporter::doMesh(const UnsignedInt id, const UnsignedInt level) { return _in->mesh(id, level); }
 
-MeshAttribute AnySceneImporter::doMeshAttributeForName(const Containers::StringView name) { return _in->meshAttributeForName(name); }
-Containers::String AnySceneImporter::doMeshAttributeName(const UnsignedShort id) { return _in->meshAttributeName(meshAttributeCustom(id)); }
+MeshAttribute AnySceneImporter::doMeshAttributeForName(const Containers::StringView name) {
+    /* This API can be called even if no file is opened, in that case return
+       an invalid ID */
+    return _in ? _in->meshAttributeForName(name) : MeshAttribute{};
+}
+Containers::String AnySceneImporter::doMeshAttributeName(const UnsignedShort id) {
+    /* This API can be called even if no file is opened, in that case return
+       an invalid ID */
+    return _in ? _in->meshAttributeName(meshAttributeCustom(id)) : Containers::String{};
+}
 
 #ifdef MAGNUM_BUILD_DEPRECATED
 CORRADE_IGNORE_DEPRECATED_PUSH

src/MagnumPlugins/AnySceneImporter/AnySceneImporter.h

@@ -146,6 +146,10 @@ Calls to the @ref animation(), @ref scene(), @ref light(), @ref camera(),
 and corresponding count-/name-related functions are then proxied to the
 concrete implementation. The @ref close() function closes and discards the
 internally instantiated plugin; @ref isOpened() works as usual.
+
+While the @ref meshAttributeName() and @ref meshAttributeForName() APIs can
+be called without a file opened, they return an empty string or an invalid
+attribute in that case.
 */
 class MAGNUM_ANYSCENEIMPORTER_EXPORT AnySceneImporter: public AbstractImporter {
     public: