NPE in FbxObject.unsupportedConnectObjectProperty()

[stephengold]

I'm testing current "master" branch with FBX assets downloaded from CGtrader, Mixamo, and Sketchfab.
Attempting to import a certain FBX file causes the following crash:

Jan 29, 2023 10:11:35 AM com.jme3.app.LegacyApplication handleError
SEVERE: Uncaught exception thrown in Thread[jME3 Main,5,main]
java.lang.NullPointerException: Cannot invoke "com.jme3.scene.plugins.fbx.obj.FbxObject.getFullClassName()" because "object" is null
	at com.jme3.scene.plugins.fbx.obj.FbxObject.unsupportedConnectObjectProperty(FbxObject.java:134)
	at com.jme3.scene.plugins.fbx.anim.FbxAnimCurveNode.connectObjectProperty(FbxAnimCurveNode.java:129)
	at com.jme3.scene.plugins.fbx.FbxLoader.connectObjects(FbxLoader.java:261)
	at com.jme3.scene.plugins.fbx.FbxLoader.loadData(FbxLoader.java:163)
	at com.jme3.scene.plugins.fbx.FbxLoader.load(FbxLoader.java:110)
	at com.jme3.asset.DesktopAssetManager.loadLocatedAsset(DesktopAssetManager.java:272)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:388)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:439)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:444)
	at jme3test.helloworld.HelloJME3.loadModel(HelloJME3.java:88)
	at jme3test.helloworld.HelloJME3.simpleInitApp(HelloJME3.java:80)
	at com.jme3.app.SimpleApplication.initialize(SimpleApplication.java:240)
	at com.jme3.system.lwjgl.LwjglAbstractDisplay.initInThread(LwjglAbstractDisplay.java:139)
	at com.jme3.system.lwjgl.LwjglAbstractDisplay.run(LwjglAbstractDisplay.java:221)
	at java.base/java.lang.Thread.run(Thread.java:833)

The crash occurs because in FbxLoader.connectObjects(), the child object (with ID 67443872) doesn't exist in objectMap:

jmonkeyengine/jme3-plugins/src/fbx/java/com/jme3/scene/plugins/fbx/FbxLoader.java

Lines 255 to 264 in 7a5d085

	} else if (type.equals("OP")) {
	childId = FbxId.create(el.properties.get(1));
	parentId = FbxId.create(el.properties.get(2));
	String propName = (String) el.properties.get(3);
	FbxObject child = objectMap.get(childId);
	FbxObject parent = objectMap.get(parentId);
	parent.connectObjectProperty(child, propName);
	} else {
	logger.log(Level.WARNING, "Unknown connection type: {0}. Ignoring.", type);
	}

At the very least, the FBX loader could fail more gracefully, providing clues as to where the problem lies. Probably the best solution would be to warn, skip, and continue.