LDrawLoader: Fix getMainEdgeMaterial() (#23334)

examples/jsm/loaders/LDrawLoader.js

@@ -2125,7 +2125,8 @@ class LDrawLoader extends Loader {
 
 	getMainEdgeMaterial() {
 
-		return this.getMaterial( MAIN_EDGE_COLOUR_CODE );
+		const mainMat = this.getMainMaterial();
+		return mainMat && mainMat.userData ? mainMat.userData.edgeMaterial : null;
 
 	}