GltfLoader sourceIndex NPE on SheenWoodLeatherSofa model

[stephengold]

SheenWoodLeatherSofa.gltf was added to the glTF-Sample-Assets test suite in May 2024.
https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/SheenWoodLeatherSofa

When I try to load it using jme3-plugins, I get warnings followed by a crash:

Mar 05, 2025 11:05:51 AM com.jme3.scene.plugins.gltf.CustomContentManager readExtension
WARNING: Could not find loader for extension KHR_materials_sheen
Mar 05, 2025 11:05:51 AM com.jme3.scene.plugins.gltf.CustomContentManager readExtension
WARNING: Could not find loader for extension KHR_materials_specular
com.jme3.asset.AssetLoadException: An error occurred loading SheenWoodLeatherSofa/glTF/SheenWoodLeatherSofa.gltf
	at com.jme3.scene.plugins.gltf.GltfLoader.loadFromStream(GltfLoader.java:181)
	at com.jme3.scene.plugins.gltf.GltfLoader.load(GltfLoader.java:107)
	at com.jme3.asset.DesktopAssetManager.loadLocatedAsset(DesktopAssetManager.java:274)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:390)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:441)
	at com.github.stephengold.wrench.test.CompareLoaders.loadModel(CompareLoaders.java:889)
	at com.github.stephengold.wrench.test.CompareLoaders.loadModel(CompareLoaders.java:855)
	at com.github.stephengold.wrench.test.CompareLoaders.onAction(CompareLoaders.java:496)
	at jme3utilities.ui.DefaultInputMode.onAction(DefaultInputMode.java:186)
	at com.jme3.input.InputManager.invokeActions(InputManager.java:174)
	at com.jme3.input.InputManager.onKeyEventQueued(InputManager.java:474)
	at com.jme3.input.InputManager.processQueue(InputManager.java:871)
	at com.jme3.input.InputManager.update(InputManager.java:923)
	at com.jme3.app.LegacyApplication.update(LegacyApplication.java:810)
	at com.jme3.app.SimpleApplication.update(SimpleApplication.java:249)
	at com.jme3.system.lwjgl.LwjglWindow.runLoop(LwjglWindow.java:707)
	at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:797)
	at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because "sourceIndex" is null
	at com.jme3.scene.plugins.gltf.GltfLoader.readTexture(GltfLoader.java:759)
	at com.jme3.scene.plugins.gltf.GltfLoader.readTexture(GltfLoader.java:739)
	at com.jme3.scene.plugins.gltf.GltfLoader.readMaterial(GltfLoader.java:650)
	at com.jme3.scene.plugins.gltf.GltfLoader.readMeshPrimitives(GltfLoader.java:473)
	at com.jme3.scene.plugins.gltf.GltfLoader.readNode(GltfLoader.java:252)
	at com.jme3.scene.plugins.gltf.GltfLoader.readChild(GltfLoader.java:301)
	at com.jme3.scene.plugins.gltf.GltfLoader.readScenes(GltfLoader.java:218)
	at com.jme3.scene.plugins.gltf.GltfLoader.loadFromStream(GltfLoader.java:155)
	... 17 more

I discovered this issue while testing JME v3.8.0-alpha4. However, the behavior hasn't changed since at least JME v3.2.4-stable, so I don't consider this a regression.