         @goto_revision()
 
     render_diff_slider: =>
+        if not @syncstring?
+            return
         @length = @syncstring.all_versions().length
         @revision_num = @length - 1
         # debounce actually setting the document content just a little

            return
        if not num1?
            num1 = @revision_num1 ? Math.max(0, Math.floor(num2/2))
        versions = @syncstring.all_versions()
        if not versions?
            # not yet initialized
            return
        time1 = versions[num1]
        if not time1?

        @view_doc?.hide()

    load_full_history: (cb) =>
        if not @syncstring?
            return
        n = @syncstring.all_versions().length
        @syncstring.load_full_history (err) =>
            if err
