fix: guard close user_can_summarize?

course on discussion topic might not be a course
but an account, for topics on account lvl groups
this fix prevents dt crashes

fixes VICE-4534
flag=discussion_summary

Test plan
- Create an account level group
- In the group context create a discussion topic
- Enable discussion_summary feature flag
- Open the topic, app should not crash

Change-Id: I3ddc7cac72aad7ed0d67150470192fbc34713cd6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/355551
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Ádám Molnár <adam.molnar@instructure.com>
Reviewed-by: Omar Soto-Fortuño <omar.soto@instructure.com>
QA-Review: Ádám Molnár <adam.molnar@instructure.com>
Product-Review: Richard Zana <rzana@instructure.com>

app/models/discussion_topic.rb

@@ -1427,6 +1427,12 @@ class DiscussionTopic < ActiveRecord::Base
       return false
     end
 
+    # course can be an account in case the topic context is group
+    # and the group context is account
+    unless course.is_a?(Course)
+      return false
+    end
+
     course.feature_enabled?(:discussion_summary) && (
       course.user_is_instructor?(user) || course.grants_right?(user, :read_as_admin)
     )

spec/models/discussion_topic_spec.rb

@@ -3590,6 +3590,20 @@ describe DiscussionTopic do
       expect(@topic.user_can_summarize?(@observer)).to be false
       expect(@topic.user_can_summarize?(@student)).to be false
     end
+
+    it "does not crash if the topic is in the context of a group with account context" do
+      account = @course.root_account
+      account.enable_feature!(:discussion_summary)
+      group = account.groups.create!
+      topic = group.discussion_topics.create!(title: "topic")
+
+      expect(topic.user_can_summarize?(@teacher)).to be false
+      expect(topic.user_can_summarize?(@ta)).to be false
+      expect(topic.user_can_summarize?(@admin)).to be false
+      expect(topic.user_can_summarize?(@designer)).to be false
+      expect(topic.user_can_summarize?(@observer)).to be false
+      expect(topic.user_can_summarize?(@student)).to be false
+    end
   end
 
   describe "low_level_locked_for?" do