TypeError: 'NoneType' object is not iterable

[admin]

How to Reproduce

While doing a GET operation on /admin/ticket/template_admin, Trac issued an internal error.

(please provide additional details here)

Request parameters:

{'cat_id': u'ticket', 'panel_id': u'template_admin', 'path_info': None}

User agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.154 Safari/537.36

System Information

Trac	1.0.1
Babel	0.9.6
Genshi	0.6 (without speedups)
mod_wsgi	3.5 (WSGIProcessGroup WSGIApplicationGroup %{GLOBAL})
pysqlite	2.6.3
Python	2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)]
setuptools	3.4.3
SQLite	3.7.6.2
Subversion	1.8.3 (r1516576)
jQuery	1.7.2

Enabled Plugins

autocompleteusers	0.4.2dev
gridmodify	0.1.6dev
ticketextplugin	0.3.2
trac-jsgantt	0.10
tracaccountmanager	0.3.2
traccustomfieldadmin	0.2.8
tracdatefield	3.0.0dev
tracganttcalendarplugin	0.6.2-r843
tracmastertickets	3.0.4
tracpermredirect	2.0

Python Traceback

Traceback (most recent call last):
  File "build\bdist.win32\egg\trac\web\main.py", line 497, in _dispatch_request
    dispatcher.dispatch(req)
  File "build\bdist.win32\egg\trac\web\main.py", line 214, in dispatch
    resp = chosen_handler.process_request(req)
  File "build\bdist.win32\egg\trac\admin\web_ui.py", line 125, in process_request
    path_info)
  File "build\bdist.win32\egg\ticketext\template_admin.py", line 67, in render_admin_panel
    self._process_read(req, page_param)
  File "build\bdist.win32\egg\ticketext\template_admin.py", line 91, in _process_read
    for cf in customfields:
TypeError: 'NoneType' object is not iterable

[Jun Omae]

A duplicate of #8809.