There is something wrong with this method (TMP_MaterialManager.ClearMaterials())

Unity Engine
1 
        public static void ClearMaterials()
        {
            if (m_materialList.Count == 0)
            {
                Debug.Log("Material List has already been cleared.");
                return;
            }

            for (int i = 0; i < m_materialList.Count; i++)
            {
                //Material baseMaterial = m_materialList[i].baseMaterial;
                Material stencilMaterial = m_materialList[i].stencilMaterial;

                Object.DestroyImmediate(stencilMaterial);
                m_materialList.RemoveAt(i);
            }
        }

It should be:

        public static void ClearMaterials()
        {
            if (m_materialList.Count == 0)
            {
                Debug.Log("Material List has already been cleared.");
                return;
            }

            for (int i = 0; i < m_materialList.Count; i++)
            {
                //Material baseMaterial = m_materialList[i].baseMaterial;
                Material stencilMaterial = m_materialList[i].stencilMaterial;

                Object.DestroyImmediate(stencilMaterial);
                //m_materialList.RemoveAt(i);
            }
            m_materialList.Clear();
        }
2

This function should no longer be referenced / used in the latest release of the TMP package.

Having said that, I concur with your change and have revised this function as such (even if it is no longer reference by the code in the TMP package).