fix a bug in asset modifier that would try to modify null materials

Assets/ScriptableRenderPipeline/MobileRenderPipeline/ClassicDeferred/ClassicDeferredPipeline.cs

 		foreach (Renderer _renderer in _renderers) {
 			Material[] _materials = _renderer.sharedMaterials;
 			foreach (Material _material in _materials) {
+				if (_material == null)
+					continue;
+				
+
 			}
 		}
 	}