dock_applet.py crashed with AttributeError in button_release(): 'NoneType' object has no attribute 'activate'

[Stevie Card]

$ lsb_release -rd
Description: Ubuntu Xenial Xerus (development branch)
Release: 16.04

$ apt-cache policy mate-dock-applet
mate-dock-applet:
  Installed: 0.67-2

I'm currently running Ubuntu in virtualbox without addons, if that makes a difference. I was switching between different layouts when the crash occurred I had just selected the Mutiny layout. Everything seemed just fine however.

ProblemType: Crash
DistroRelease: Ubuntu 16.04
Package: mate-dock-applet 0.67-2
ProcVersionSignature: Ubuntu 4.3.0-7.18-generic 4.3.3
Uname: Linux 4.3.0-7-generic x86_64
ApportVersion: 2.20-0ubuntu3
Architecture: amd64
CurrentDesktop: MATE
Date: Wed Feb 24 02:56:52 2016
ExecutablePath: /usr/lib/mate-applets/mate-dock-applet/dock_applet.py
InstallationDate: Installed on 2016-02-24 (0 days ago)
InstallationMedia: Ubuntu-MATE 16.04 LTS "Xenial Xerus" - Alpha amd64 (20160127.1)
InterpreterPath: /usr/bin/python3.5
ProcCmdline: python3 /usr/lib/mate-applets/mate-dock-applet/dock_applet.py
ProcEnviron:
 SHELL=/bin/bash
 XDG_RUNTIME_DIR=<set>
 PATH=(custom, no user)
 LANGUAGE=en_US
 LANG=en_US.UTF-8
PythonArgs: ['/usr/lib/mate-applets/mate-dock-applet/dock_applet.py']
SourcePackage: mate-dock-applet
Title: dock_applet.py crashed with AttributeError in button_release(): 'NoneType' object has no attribute 'activate'
Traceback:
 Traceback (most recent call last):
   File "/usr/lib/mate-applets/mate-dock-applet/dock_win_list.py", line 326, in button_release
     wnck_ws.activate(0)
 AttributeError: 'NoneType' object has no attribute 'activate'
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo

[cyril rosik (roscyril-deactivatedaccount)]

crash mate-panel
change display in mate tweak and ubuntu-mate-welcome

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Norbert (nrbrtx)]

Can you reproduce this bug with newer version of Ubuntu MATE?

[Launchpad Janitor (janitor)]

[Expired for mate-dock-applet (Ubuntu) because there has been no activity for 60 days.]