Are you going to repair this soon ?
Happens about 5 times a day.
See also bug report #1549971
Hi,
I’m the developer of the applet and wasn’t aware of this particular problem. However, now that I am, I’ll look into it. 
By the way, I can’t find the bug report on Launchpad by searching for the report number you quoted or for ‘mate-dock-applet’. Could you provide me with a direct link?
Thanks.
Thank you Robin for your help, by the way I am using compiz and have 6 workspaces in use.
The bug report on Launchpad is in state privat, so you do not have permission to read it, strange thing 
The direct link is here, maybe M. Wimpress has permission to read ?
Therefore I copy-pasted from Launchpad:
dock_applet.py crashed with UnboundLocalError in get_cmdline_from_pid(): local variable 'line' referenced before assignment Edit
Bug #1549971 reported by Peter Bauer on 2016-02-25
160
This bug affects you 
Affects Status Importance Assigned to Milestone
mate-dock-applet (Ubuntu)
New
Medium
Unassigned  Edit
Also affects project (?) Also affects distribution/package
Edit
Bug Description
Just testing the mutiny mate layout with 3 terminal instaces on the panel.
-open/close terminal 1 to 3 from panel and change input focus from 1 to 3
-crash happens
ProblemType: Crash
DistroRelease: Ubuntu 16.04
Package: mate-dock-applet 0.67-2
ProcVersionSignature: Ubuntu 4.4.0-8.23-generic 4.4.2
Uname: Linux 4.4.0-8-generic x86_64
ApportVersion: 2.20-0ubuntu3
Architecture: amd64
CurrentDesktop: MATE
Date: Thu Feb 25 21:18:35 2016
ExecutablePath: /usr/lib/mate-applets/mate-dock-applet/dock_applet.py
InstallationDate: Installed on 2016-01-22 (34 days ago)
InstallationMedia: Ubuntu-MATE 16.04 LTS "Xenial Xerus" - Alpha amd64 (20160103)
InterpreterPath: /usr/bin/python3.5
ProcCmdline: python3 /usr/lib/mate-applets/mate-dock-applet/dock_applet.py
PythonArgs: ['/usr/lib/mate-applets/mate-dock-applet/dock_applet.py']
SourcePackage: mate-dock-applet
Title: dock_applet.py crashed with UnboundLocalError in get_cmdline_from_pid(): local variable 'line' referenced before assignment
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo