wi-fi applet suddenly disappears (nm-applet stops)

The wi-fi applet at the top of the screen disappears with no warning.
When I start it again with `nm-applet` via the terminal, it often crashes again (as shown below).

    $ nm-applet
    ** Message: applet now removed from the notification area
    ** Message: PID 16587 (we are 14963) sent signal 15, shutting down...

Description: Ubuntu 12.04.1 LTS
Release: 12.04

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: network-manager-gnome 0.9.4.1-0ubuntu2
ProcVersionSignature: Ubuntu 3.2.0-31.50-generic 3.2.28
Uname: Linux 3.2.0-31-generic x86_64
ApportVersion: 2.0.1-0ubuntu13
Architecture: amd64
Date: Sun Sep 23 16:18:19 2012
ExecutablePath: /usr/bin/nm-applet
IfupdownConfig:
 auto lo
 iface lo inet loopback
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
IpRoute:
 default via 192.168.1.1 dev wlan0 proto static
 169.254.0.0/16 dev wlan0 scope link metric 1000
 192.168.1.0/24 dev wlan0 proto kernel scope link src 192.168.1.111 metric 2
NetworkManager.state:
 [main]
 NetworkingEnabled=true
 WirelessEnabled=true
 WWANEnabled=true
SourcePackage: network-manager-applet
UpgradeStatus: Upgraded to precise on 2012-05-01 (145 days ago)
nmcli-dev:
 DEVICE TYPE STATE DBUS-PATH
 wlan0 802-11-wireless connected /org/freedesktop/NetworkManager/Devices/1
 eth0 802-3-ethernet unavailable /org/freedesktop/NetworkManager/Devices/0
nmcli-nm:
 RUNNING VERSION STATE NET-ENABLED WIFI-HARDWARE WIFI WWAN-HARDWARE WWAN
 running 0.9.4.0 connected enabled enabled enabled enabled enabled