I'd been using Mate on Ubuntu 18.04 with the mojave-gtk-theme and the main menu configured in control center appeared in the middle of the menu bar. After updating to the latest version, the main menu (Applications/System ) disappeared. I've searched and not found anything about this.
How do I get it back? Or, how do I get a menu listing desktop applications that are installed like that menu showed?
upgrades sometimes corrupt your panel layout. You may choose some preconfigured panel layout in MATE Tweak, but this will reset all changes you have currently applied to your panel. Or you may simply add a menu applet (4 types are shipped with Ubuntu MATE) to your panel via panel context menu.
You can also save your panel layout once you have what you like and reactivate it if lost.
System>Control Center>MATE Tweak>Panel>Select a Panel layout>save as
But... his layout is broken. There's a very small chance that saving and re-enabling a broken layout will fix the problem. I suggest selecting preconfigured and tested layout instead.