I had this issue when reloading a scene or switching from one to another. The other solutions didn't work for me. In Unity 6, this is what worked:

public class ReverseMask : Image
{
    private Material _customMaterial;

    public override Material materialForRendering
    {
        get
        {
            if (_customMaterial == null)
            {
                _customMaterial = new Material(base.materialForRendering);
                _customMaterial.SetFloat("_StencilComp", (float)CompareFunction.NotEqual);
            }

            return _customMaterial;
        }
    }
}