Undefined artefact in thumb

[CosmoFruit]

if src for thumb is failed we look this:

can you fix undefined text in html?

(fall back and blue contur is my custom template)

Which versions are you using for the following packages?

Gallery: 3.0.1

[MurhafSousli]

This undefined isn't caused by thumb src failure, it is about the loadingIcon that is undefined, anyway you can use CSS

gallery-thumb {
   .g-loading {
       display: none;
   }
}