Menu theme location missing

Hello everyone! First of all sorry me for my bad English but isn’t my native language so I’ll do as best as possible to explain my problem.

I’m running WordPress 4.5.3 and Max Mega Menu plugin. When I’ve tried to modify a menu from Appearance -> Menus admin screen (to add pages or links), the theme location of that menu was lost and the front-end of the site displays an alphabetical list of all elements of the menu without any style (lost the CSS too).

I only have one menu on the site. Is there a way to “remember” the location of the menu? Now, I’ve to choose it every time I want to update it. Thanks in advance! =)

(The theme is a personal one and I’ve added the function to register the menu on functions.php)

https://wordpress.org/plugins/megamenu/