Clicking 'edit menu' link causes PHP warning 'Trying to access array offset on value of type bool'

In the function og_menu_form_menu_edit_menu_alter of the module og_menu.module, $ogm is false at line 526 if users edit regular menus (i.e. non-OG menus). To fix this bug, I have modified the code to check if $ogm is not false before retrieving its array item. Please see my attached patch. Thank you.

I've reviewed this code with franceslui and it seems to be overstepping when it doesn't have a group id. Thanks @franceslui for posting the patch.