RenderTexture for computer shader is being null on material

Hi,guys I write a compute shader for a rendertexure and set this rendertexture to be a global texture( Shader.SetGlobalTexture()),but I find that if I do not select it in editor,it will be null on material,this is my script:

 public ComputeShader computeShader;
     public int texSize = 256;
     [Range(5,256)]
     public float renderRange = 5;
 
     public RenderTexture rendTex;
     public RenderTexture computeTex;
 
 
     Camera topCamera;
     int kernelHandle;
     
     // Start is called before the first frame update
     void Start()
     {
         
         Init();
     }
 
     // Update is called once per frame
     void Update()
     {
         RunShader();
     }
 
     private void OnDisable()
     {
         rendTex.Release();
         computeTex.Release();
     }
 
     private void Init()
     {
         topCamera = this.GetComponent<Camera>();
 
         kernelHandle = computeShader.FindKernel("CSMain");
 
         rendTex = new RenderTexture(texSize, texSize, 16,RenderTextureFormat.Depth);
         computeTex = new RenderTexture(texSize, texSize, 0);
         computeTex.enableRandomWrite = true;
         computeTex.name = "TopRenderTexture";
         //rendTex.Create();
         //computeTex.Create();
 
         if (topCamera != null)
         {
             topCamera.targetTexture = rendTex;
             topCamera.enabled = false;
             topCamera.orthographicSize = renderRange;
         }
 
         computeShader.SetTexture(kernelHandle, "Result", computeTex);
         computeShader.SetTexture(kernelHandle, "TopDepthTex", rendTex);
         Shader.SetGlobalTexture("_TopRenderTexture", computeTex);
     }
     void RunShader()
     {
         UpdateTopCamera();
 
         topCamera.Render();
         computeShader.Dispatch(kernelHandle, texSize / 8, texSize / 8, 1);
     }
     void UpdateTopCamera()
     {
         if (topCamera != null)
         {
             topCamera.orthographicSize = renderRange;
         }
     }

The compute shader is working perfect,but when I run this script,the "TopRenderTexture"is null at first on material: Then I double click this RenderTexture in script Inspector,show rendertexture's inspector,and the result will be right: It seems like this rendertexture must be rendered on my editor window,then it's pointer will correctly connect to the materials?How can I fix it?Thanks for answering.