UI穿插spine对象问题

spine 文件穿插在list中， 按照
http://www.fairygui.com/guide/unity/insert3d.html
加入裁剪。 但是下面这段代码会报错

if (clearStencil)

				{

					int cnt = _renderers.Length;

					for (int i = 0; i < cnt; i++)

					{

						Renderer r = _renderers[i];

						if (r != null)

						{

							r.sharedMaterial.SetInt("_StencilComp", (int)UnityEngine.Rendering.CompareFunction.Always);

							r.sharedMaterial.SetInt("_Stencil", 0);

							r.sharedMaterial.SetInt("_StencilOp", (int)UnityEngine.Rendering.StencilOp.Keep);

							r.sharedMaterial.SetInt("_StencilReadMask", 255);

							r.sharedMaterial.SetInt("_ColorMask", 15);

						}

					}

				}

有一帧 r.sharedMaterial 是null的。之后都是好的 这里应该要加判断的吧