The issue is not related to the message you have. You can ignore the message as it is there because you do not have a purchase key of the Envira Gallery. But that plugin is bundled with our theme and you do not need one. For more information kindly read the bundled version section of the article below:
This will not happen in most modern browsers but there are plenty of them yet calculating the height 0.
So your case is one of the general rules of thumb. For example, you should not also add the gallery to a tab or accordion as those elements are hidden initially. For the tab, you can add the gallery to the first tab as it shows on load.
I hope I could explain the case in details and reason behind the Envira Gallery problem when it goes inside the off-canvas.