ClassCastException if MTL loader can not find material texture

JME throws a ClassCastException if the MTL loader can not find the material texture key. Code:

        this.box = assetManager.loadModel("assets/models/tiles/box-1.obj");
        Material mat1 = assetManager.loadMaterial("assets/materials/tiles/loam/loam-1-1.mtl")
        mat1.setTexture("textures/loam/loam-1-1.png", assetManager.loadTexture("assets/textures/tiles/loam/loam-1-1.png"));
        box.setMaterial(mat1);
        return box

See the code in https://github.com/jMonkeyEngine/jmonkeyengine/blob/v3.2.4-stable/jme3-core/src/plugins/java/com/jme3/scene/plugins/MTLLoader.java#L186

            logger.log(Level.WARNING, "Cannot locate {0} for material {1}", new Object[]{texKey, key});
            texture = new Texture2D(PlaceholderAssets.getPlaceholderImage(assetManager));
            texture.setWrap(WrapMode.Repeat);
            texture.setKey(key);

You set the MaterialKey to a texture object. It will throw a CCEx on https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/texture/Texture.java#L427

    public void setKey(AssetKey key){
        this.key = (TextureKey) key;
    }

Error:

java.lang.ClassCastException: class com.jme3.asset.MaterialKey cannot be cast to class com.jme3.asset.TextureKey (com.jme3.asset.MaterialKey and com.jme3.asset.TextureKey are in unnamed module of loader 'app')
	at com.jme3.texture.Texture.setKey(Texture.java:427)
	at com.jme3.scene.plugins.MTLLoader.loadTexture(MTLLoader.java:186)
	at com.jme3.scene.plugins.MTLLoader.readLine(MTLLoader.java:232)
	at com.jme3.scene.plugins.MTLLoader.load(MTLLoader.java:304)

Version: 3.2.4-stable

Start in jme3-core/src/plugins/java/com/jme3/scene/plugins/MTLLoader.java at the missing-texture handling around line 186, then inspect com/jme3/texture/Texture.java at setKey. Reproduce loading an MTL file whose material texture cannot be found; done means this case no longer throws ClassCastException and the existing placeholder behavior remains usable.