fix: Notify observers with correct material parameter (instead of mesh)

src/babylon.scene.ts

@@ -3232,7 +3232,7 @@ module BABYLON {
          */
         public addMaterial(newMaterial: Material): void {
             this.materials.push(newMaterial);
-            this.onNewMaterialAddedObservable.notifyObservers(newMesh);
+            this.onNewMaterialAddedObservable.notifyObservers(newMaterial);
         }
 
         /**