fix wrong tabs layout on validation error after page reload

[jmariller]

This fixes #5980

[javiereguiluz]

Thanks @jmariller.

I tried to solve this "in a better way". by trying to use only form options, etc. I just wasted 1 hour. I can't update the form options in the post_submit event (and in Symfony Docs I can't find much information about form events), I don't know if this could be solved with form attributes (Symfony Docs doesn't explain them), etc.

So, your proposal is really the best way to solve this. Let's merge it. Thanks