I found a problem with this query from lib/accesslib.php function get_roles_with_cap_in_context:
  $sql = "SELECT rc.id, rc.roleid, rc.permission, ctx.depth
              FROM {role_capabilities} rc
              JOIN {context} ctx ON ctx.id = rc.contextid
              JOIN {capabilities} cap ON rc.capability = cap.name
             WHERE rc.capability = :cap AND ctx.id IN ($ctxids)
          ORDER BY rc.roleid ASC, ctx.depth DESC";
 
The role_capabilities table should be joined with the role table because when a role is removed, the roleid data related column in the role_capabilities table remains. As a quick solution to address the missing cascade deletion, I suggest adding JOIN {role} r ON r.id = rc.roleid to the query.
 
I came across with the issue described above by clicling in settings tab in module forum after remove some user roles:

 
Exception - role_get_name(): Argument #1 ($role) must be of type stdClass, null given, called in [dirroot]/lib/accesslib.php on line 4634