Module added to filter and then disabled while filter is still in SESSION variable causes warning

[yorkshire-pudding]

This is an edge case that I encountered while testing a module I was porting, but it is feasible that it could happen in real world usage.

Steps to reproduce

1. Enable a module with permissions
2. Go to permission screen
3. Select only that module and apply the filter
4. Disable that module
5. Return to the permission screen (or reload it)

Expected result
Page loads with all the modules, or otherwise handles this.

Actual result
Warning:

Warning: Undefined array key "#description" in theme_user_admin_permissions() (line 133 of /app/docroot/core/modules/user/user.theme.inc).

If devel is loaded, then can lock the page while it builds the Krumo tree.