Fix doi variable not being initialized in the side-bar box

invenio_theme_tuw/templates/invenio_theme_tuw/details/side_bar.html

@@ -28,6 +28,7 @@
       <div class="one column row">
         <div class="column left aligned">
           <p class="text-muted font-small">
+          {%- set doi = metadata.identifiers|tuw_doi_identifier -%}
           {%- if doi -%}
             DOI: <a href="https://doi.org/{{ doi }}">{{ doi }}</a>
           {%- else -%}