plasma-widget-menubar crashes plasma-desktop frequently

Almost every time I run Zim (https://launchpad.net/zim), Plasma crashes. This did not happen before I upgraded from Oneiric to Precise. Now it happens constantly. This is very frustrating.

Sometimes Plasma freezes for a few minutes before it crashes, and other times it crashes right away.

After Plasma restarts, it works fine, even though I continue using Zim, including hiding and restoring the Zim window.

Sometimes the Zim tray icon doesn't appear in the tray, instead leaving an empty space for the icon. Other times it works fine. I don't know if this behavior is related. None of this was a problem with Oneiric.

I installed 341 MB of Plasma debug symbols from plasma-workspace-dbg, but it still says "The generated crash information is probably not useful." It says there are no symbols for /usr/bin/plasma-desktop nor
/usr/lib/kde4/plasma_applet_menubar.so. The "Install debug symbols" button doesn't find any packages to install. Apparently there are no debug symbols for these files in the archive. (How there could be none for /usr/bin/plasma-desktop bewilders me.)

The backtrace of the crashed thread is:

Thread 1 (Thread 0xb76f1a40 (LWP 19777)):
[KCrash Handler]
#7 0x47d50324 in QVariant::userType (this=0xf) at kernel/qvariant.cpp:1784
#8 0xb5366eb0 in ?? () from /usr/lib/kde4/plasma_applet_menubar.so
#9 0xb535cb32 in ?? () from /usr/lib/kde4/plasma_applet_menubar.so
#10 0xb535cbec in ?? () from /usr/lib/kde4/plasma_applet_menubar.so
#11 0x47eadf98 in QDBusConnectionPrivate::deliverCall (this=0x9db04d0, object=0x9fd10f0, msg=..., metaTypes=..., slotIdx=15) at qdbusintegrator.cpp:947
#12 0x47eb81ed in QDBusCallDeliveryEvent::placeMetaCall (this=0xb02b4a8, object=0x9fd10f0) at qdbusintegrator_p.h:103
#13 0x47d471bb in QObject::event (this=0x9fd10f0, e=0xb02b4a8) at kernel/qobject.cpp:1195
#14 0x4851c654 in notify_helper (e=0xb02b4a8, receiver=0x9fd10f0, this=0x9ddfce0) at kernel/qapplication.cpp:4559
#15 QApplicationPrivate::notify_helper (this=0x9ddfce0, receiver=0x9fd10f0, e=0xb02b4a8) at kernel/qapplication.cpp:4531
#16 0x48521a8d in QApplication::notify (this=0xb02b4a8, receiver=0x9fd10f0, e=0xb02b4a8) at kernel/qapplication.cpp:4288
#17 0x422894e1 in KApplication::notify (this=0x9daed98, receiver=0x9fd10f0, event=0xb02b4a8) at ../../kdeui/kernel/kapplication.cpp:311
[...]

Again, none of this ever happened with Oneiric. So far, since upgrading to Precise, I am experiencing this and two other regular crashes, one of which freezes X completely, requiring me to kill it and all child processes. Right now, I wish I hadn't upgraded, but how could I have known about these bugs if I hadn't? Would anyone else report them? Would a new user even continue using Kubuntu if he experienced such crashes? And all of this is in an LTS release. :(

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: plasma-widget-menubar 0.1.17-0ubuntu1
Uname: Linux 3.3.6-pf-adp+ i686
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu7
Architecture: i386
Date: Wed May 16 19:32:10 2012
ProcEnviron:
 LANGUAGE=
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: plasma-widget-menubar
UpgradeStatus: Upgraded to precise on 2012-05-06 (10 days ago)