I think it's better to check like this:
$birra = isset( $passaggio['et2018-nome_birra'] ) ? $passaggio['et2018-nome_birra'] : '';
So the variable is always available and won't display any warning if you do echo $birra;
.
PS: You can get the group value your way or use the helper function like this:
$passaggio = rwmb_meta( 'gruppo_birra' );
In your code, please make sure the global $post
is available. Otherwise it won't work. You can try this instead:
$passaggio = get_post_meta( get_the_ID(), 'gruppo_birra', true );