ubiquity crashed with AttributeError in on_link_clicked(): 'NoneType' object has no attribute 'split'

ubiquity crashed just after click on "update this installer" in the first step.

ProblemType: Crash
DistroRelease: Ubuntu 12.04
Package: ubiquity 2.10.4
ProcVersionSignature: Ubuntu 3.2.0-20.33-generic 3.2.12
Uname: Linux 3.2.0-20-generic x86_64
ApportVersion: 1.95-0ubuntu1
Architecture: amd64
CasperVersion: 1.312
Date: Thu Apr 19 03:12:05 2012
ExecutablePath: /usr/lib/ubiquity/bin/ubiquity
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash -- maybe-ubiquity
InterpreterPath: /usr/bin/python2.7
LiveMediaBuild: Ubuntu 12.04 LTS "Precise Pangolin" - Beta amd64 (20120328)
ProcCmdline: /usr/bin/python /usr/lib/ubiquity/bin/ubiquity --greeter --only
ProcEnviron: Error: [Errno 13] Permission denied: '/proc/3041/environ'
PythonArgs: ['/usr/lib/ubiquity/bin/ubiquity', '--greeter', '--only']
SourcePackage: ubiquity
Title: ubiquity crashed with AttributeError in on_link_clicked(): 'NoneType' object has no attribute 'split'
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: