This issue turned up in [#3657]. During migration, the svnsync failed and left the repo in a bad state. Here's a traceback:
Module /var/local/allura/Allura/allura/templates/widgets/repo/tree_widget.html:3 in top-level template code
<< {% import 'allura:templates/jinja_master/lib.html' as lib with context %}
<table>
{% set ls = list(tree.ls()) %}
<thead>
<tr>
>> {% set ls = list(tree.ls()) %}
UndefinedError: 'None' has no attribute 'ls'
This is the repo that failed: https://easyitsp.svn.sourceforge.net/svnroot/easyitsp/
Pausing this one in favor of a more urgent ticket.
Can't reproduce and the project's repo is now working correctly.