*DESCRIPTION*
After implementation of read only sessions (MDL-58018), on sites using the Classic theme it is not possible to expand some nodes of the Navigation block.  Particularly, users cannot expand course sections or courses listed under "My courses".

*STEPS TO REPLICATE*
1. Ensure that read only sessions is enabled:

$CFG->enable_read_only_sessions = true;

2. Set the site theme to Classic or enter Classic theme by URL.
3. Enter any course with multiple course sections containing content.
4. Open the browser developer tools.
5. Try to expand any of the course sections in the Navigation block

OBSERVED BEHAVIOR:
A spinning wheel is displayed next to the name of the course section but the section never expands.  On the network tab of the developer tools there is a failed request (HTTP 500) to/lib/ajax/getnavbranch.php

EXPECTED BEHAVIOR:
The course section expands within the Navigation block to display a list of activities within the section.