fix None canon output to be emptystring

archivebox/core/admin.py

@@ -97,7 +97,7 @@ class SnapshotAdmin(admin.ModelAdmin):
         canon = link.canonical_outputs()
         out_dir = Path(link.link_dir)
 
-        link_tuple = lambda link, method: (link.archive_path, canon[method], canon[method] and (out_dir / canon[method]).exists())
+        link_tuple = lambda link, method: (link.archive_path, canon[method] or '', canon[method] and (out_dir / (canon[method] or 'notdone')).exists())
 
         return format_html(
             '<span class="files-icons" style="font-size: 1.2em; opacity: 0.8">'