Groups membership from student perspective fixed

fixes CNVS-18055

Test plan:
* As a teacher create a few groups within a few group sets that have
  self signup enabled
* As a student, go to the groups page and join/switch to/leave some of
  these groups
* Ensure that the joinability of the groups in the group set update
  properly
* Ensure that this works when a group does not have a student leader

Change-Id: Ia1f52beb5dd97afc5f43ee7eb161e4ebd6f2b979
Reviewed-on: https://gerrit.instructure.com/47463
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Andrew Butterfield <abutterfield@instructure.com>

app/jsx/groups/StudentView.jsx

@@ -127,7 +127,7 @@ define([
       groupModel.set('users', _.reject(groupModel.get('users'), (u) => u.id === userId ));
       // If user was a leader, unset the leader attribute.
       var leader = groupModel.get("leader");
-      if (leader.id == userId) {
+      if (leader && leader.id == userId) {
         groupModel.set("leader", null);
       }
     },