slide theme derived theme has documentation_options blank URL_ROOT

[jodygarnett]

In updating sphinx I am running into an error with a custom theme based on slides:

[Error] TypeError: null is not an object (evaluating 'document.getElementById("documentation_options").getAttribute')

A related fix appears to be sphinx-doc/sphinx#4916

Blindly performing the layout.html fix indicated above resolves the issue:

    <script type="text/javascript" id="documentation_options" data-url_root="{{ pathto('', 1) }}" src="{{ pathto('_static/documentation_options.js', 1) }}"></script>
    <script type="text/javascript">
      var DOCUMENTATION_OPTIONS = {
        URL_ROOT:    document.getElementById("documentation_options").getAttribute('data-url_root'),
        VERSION:     '{{ release|e }}',
        COLLAPSE_INDEX: false,
        FILE_SUFFIX: '{{ '' if no_search_suffix else file_suffix }}',
        HAS_SOURCE:  {{ has_source|lower }}
      };
    </script>