CreateWithShader can only be called from the main thread

Question

Note: I encounter a strange problem. I know that as the title suggests I have to create shader/material in main thread but the same code is working with other data without using the main thread.

I working to make runtime Obj Importer async. For this reason all the things are working fine and my code almost become async but its is very slow (which is a seperate question). Now, I stuck into a strange issue that when I get new material name string other than "materail0" then, i get an exception :

UnityException: CreateWithShader can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.

Here is the code which is inside Task.run:

if (splitLine[0] == "newmtl")
            {
                string materialName = processedLine.Substring(7);
                Debug.Log("------------Material--------------");
                Debug.Log(materialName);
                if (materialName == "VB") { 
                    materialName = "material0";//work around
                }
                
                var newMtl = new Material(shader) { name = materialName };
                mtlDict[materialName] = newMtl;
                currentMaterial = newMtl;

                continue;
            }

Now if the material name is == to "materail0" then all things work fine and there is no exception as I get another name of material like "VB" I get an exception.