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

[sauih]

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

ProblemType: Crash
DistroRelease: Ubuntu 18.04
Package: mate-dock-applet 0.85-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu1
Architecture: amd64
CurrentDesktop: MATE
Date: Tue Mar 27 12:12:52 2018
ExecutablePath: /usr/lib/mate-applets/mate-dock-applet/dock_applet.py
InstallationDate: Installed on 2018-03-27 (0 days ago)
InstallationMedia: Ubuntu-MATE 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180323)
InterpreterPath: /usr/bin/python3.6
JournalErrors:
 -- Logs begin at Mon 2018-03-26 23:31:51 -03, end at Tue 2018-03-27 12:15:18 -03. --
 mar 27 08:37:08 username-R480-L-B241P1 wpa_supplicant[806]: dbus: wpa_dbus_get_object_properties: failed to get object properties: (none) none
 mar 27 08:37:08 username-R480-L-B241P1 wpa_supplicant[806]: dbus: Failed to construct signal
 mar 27 08:37:26 username-R480-L-B241P1 pulseaudio[1387]: [pulseaudio] backend-ofono.c: Failed to register as a handsfree audio agent with ofono: org.freedesktop.DBus.Error.ServiceUnknown: The name org.ofono was not provided by any .service files
 mar 27 08:37:27 username-R480-L-B241P1 spice-vdagent[1592]: Cannot access vdagent virtio channel /dev/virtio-ports/com.redhat.spice.0
ProcCmdline: python3 /usr/lib/mate-applets/mate-dock-applet/dock_applet.py
ProcEnviron:
 XDG_RUNTIME_DIR=<set>
 SHELL=/bin/bash
 LANGUAGE=pt_BR:pt:en
 PATH=(custom, no user)
 LANG=pt_BR.UTF-8
Python3Details: /usr/bin/python3.6, Python 3.6.5rc1, python3-minimal, 3.6.4-1
PythonArgs: ['/usr/lib/mate-applets/mate-dock-applet/dock_applet.py']
PythonDetails: /usr/bin/python2.7, Python 2.7.14+, python-minimal, 2.7.14-4
SourcePackage: mate-dock-applet
Title: dock_applet.py crashed with AttributeError in do_window_scroll(): 'NoneType' object has no attribute 'is_running'
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo