Add serialize_issuable helper and implement.

app/helpers/issuables_helper.rb

@@ -30,6 +30,15 @@ module IssuablesHelper
     end
   end
 
+  def serialize_issuable(issuable)
+    case issuable
+    when Issue
+      IssueSerializer.new.represent(issuable).to_json
+    when MergeRequest
+      MergeRequestSerializer.new.represent(issuable).to_json
+    end
+  end
+
   def can_add_template?(issuable)
     names = issuable_templates(issuable)
     names.empty? && can?(current_user, :push_code, @project) && !@project.private?

app/views/shared/issuable/_sidebar.html.haml

@@ -206,7 +206,7 @@
 
     :javascript
       gl.IssuableResource = new gl.SubbableResource('#{issuable_json_path(issuable)}');
-      new gl.IssuableTimeTracking('#{issuable.to_json}');
+      new gl.IssuableTimeTracking('#{serialize_issuable(issuable)}');
       new MilestoneSelect('{"namespace":"#{@project.namespace.path}","path":"#{@project.path}"}');
       new LabelsSelect();
       new WeightSelect();