erase reply if parent exists fixes #11819

test plan:
1. delete a root entry
2. load the page (it should work)

Change-Id: I7a6a27bf8253955f74c518981ade3f5b87fee972
Reviewed-on: https://gerrit.instructure.com/15332
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>

app/coffeescripts/models/Topic.coffee

@@ -103,6 +103,6 @@ define [
 
     maybeRemove: (entry) ->
       if entry.deleted and !entry.replies
-        erase entry.parent.replies, entry
+        erase entry.parent.replies, entry if entry.parent?.replies?
         delete @flattened[entry.id]