Widgets and Nav Menus components produce errors if user only has customize capability

[westonruter]

In #28605, a customize capability was introduced to allow a plugin to grant users (e.g. subscribers) access to the Customizer without having to grant all edit_theme_options. However, the Widgets and Nav Menus components currently are not fully taking this into account, resulting in errors when objects are not present as expected

There is a PHP notice:

Notice: Trying to get property of non-object in .../wp-includes/class-wp-customize-nav-menus.php on line 368

And a JS error:

Uncaught TypeError: Cannot read property 'bind' of undefined in customize-preview-widgets.js

Relates to having discrete capabilities for editing nav menus (#29213) and widgets (#31020).

[westonruter]

In 36611:

Customize: Prevent PHP notice and JS error caused by widgets and nav menus components if user only has customize capability.

Short-circuits components from initializing their hooks needlessly if current user lacks required capability.

Fixes #35895.