Use build.version_name first, in case build.version is None (#272)

* Use build.version_name first, in case build.version is None

When versions are removed, the builds remain with a `build.version = None`,
along with some additional attributes on the Build instance (for example
`build.version_name`).

This bug affected any display that listed projects/versions that listed
the most recent build.

Fixes readthedocs/readthedocs.org#11061

* Use get_version_name method instead of template logic

Author: agjohnson

readthedocsext/theme/templates/includes/elements/chips/build.html

@@ -13,16 +13,24 @@
 {% endblock chip_icon %}
 
 {% block chip_text %}
-  {# Allow for override of the text for version listing, where version is already stated #}
-  {{ text|default:build.version.verbose_name }}
+  {% comment %}
+    Allow for override of the text for version listing, where version is
+    already stated. Also use build method to get version name, in case version
+    instance was deleted at some point. The version name is stored on the build
+    in this scenario.
+  {% endcomment %}
+  {% firstof text build.get_version_name %}
 {% endblock chip_text %}
 
 {% block chip_detail_text %}
   #{{ build.id }}
 {% endblock chip_detail_text %}
 
 {% block popupcard_header %}
-  {{ text|default:build.version.verbose_name }} #{{ build.id }}
+  {% comment %}
+    This block is the same as the chip_text block above.
+  {% endcomment %}
+  {% firstof text build.get_version_name %} #{{ build.id }}
 {% endblock popupcard_header %}
 
 {% block popupcard_right %}