fix sentry error

test plan
 - specs should pass

fixes VICE-1569
flag=none

Change-Id: Ibb8d7e656e80ce52a82f5445f0d611c39163a2ce
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/266669
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Tested-by: Mauricio Ribeiro <mauricio.ribeiro@instructure.com>
Reviewed-by: Mauricio Ribeiro <mauricio.ribeiro@instructure.com>
QA-Review: Mauricio Ribeiro <mauricio.ribeiro@instructure.com>
Product-Review: Mauricio Ribeiro <mauricio.ribeiro@instructure.com>

app/models/discussion_topic.rb

@@ -648,6 +648,8 @@ class DiscussionTopic < ActiveRecord::Base
   end
 
   def child_topic_for(user)
+    return unless context.is_a?(Course)
+
     group_ids = user.group_memberships.active.pluck(:group_id) &
       context.groups.active.pluck(:id)
     child_topics.active.where(context_id: group_ids, context_type: 'Group').first

spec/models/discussion_topic_spec.rb

@@ -1962,6 +1962,13 @@ describe DiscussionTopic do
       expect(@topic.subscription_hold(@student, nil, nil)).to eql(:not_in_group_set)
     end
 
+    it "should not fail for group discussion" do
+      group = group_model(name: 'Project Group 1', group_category: @group_category, context: @course)
+      topic = group.discussion_topics.create!(title: 'hi', message: 'hey')
+      expect(topic.subscription_hold(@student, nil, nil)).to eql(:not_in_group)
+      expect(topic.child_topic_for(@student)).to be_nil
+    end
+
     it "should hold when the user is not in a group" do
       group_discussion_assignment
       expect(@topic.child_topics.first.subscription_hold(@student, nil, nil)).to eql(:not_in_group)