ERROR: No texture in the texture slot.

I am converting my sketchfab models into blend4web format.
I used textures anywhere because otherwise sketchfab rendered different with Chrome and Firefox.
Looks like blend4web does not render different so I decided to delete textures, where ther is just plain color.
Now I get this error with one material and it renders purple as missing texture.
ERROR: No texture in the texture slot. Material "red arrow".

generaator.blend
generaator.html

Looks like the texture is "stuck" - just add a new one and delete it

It happens when you delete the texture with shift, leaving it 0 users. So after reopening the file the texture will be missing, but the slot won't be deleted, it will just be empty. And Blender will do something like this

Here's a video, just in case
no_tex.mp4