The red error text "Lack of solid fuel" inside the Fuel Input Machine GUI disappears only if you have something smelting and the solid fuel is present. It should only appear if no fuel is present. Otherwise a different error text is needed like "Nothing so smelt". I have made a Fork that changes when the message appears.
I would also like if the tinker's seared tank was optional. You could just use Items like coal to heat the furnance. I tried coding that for an intuitive factorio like modpack but I am not able to.

@JaguarM (This is an old issue now, and there has been no updates here for years.) But, you could make a PR with the two changes you did, that would make it very easy for @gkbm2013 to potentially accept the changes.
And yes, this "Lack of solid fuel" issue was confusing me the other day.

It is also very confusing that the smeltery sometimes stalls if it does not have any lava, so it waits to use a tiny amount (50mb?) of lava before continuing on with solid fuel again. I'd call that a bug.