Shader > Opacity, Blending>Additive - No Render View

If you are creating a shader with just emission an use opacity value/texture and active in blending>blend mode: Additive the render in the little viewport disappear.

The only way that I found to see it again is:

+ Option1: If you desactivate the checkbox of Skybox and leve it in that way.

+ Option 2: Cloase the tool and re load the sahder again and this one will render in the viewport.