Fix 'DoGetTextExtent - invalid DC' error

most likely this error happens only on certain wx backends.
GTK3+ on GNU/Linux is apparently one of them.
The problem is described in wxWidgets issue tracker.
http://trac.wxwidgets.org/ticket/12486

[-------------------------------------------------------------------------------------------------------------------]
platform:
Linux-4.16.0-2-rt-amd64-x86_64-with-debian-buster-sid

python-version:
2.7.15

traceback:
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1605, in DispatchDebugValuesProc
self.CallWeakcallables("__tick__", "NewDataAvailable", debug_ticks)
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1585, in CallWeakcallables
function(*cargs)
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 1217, in NewDataAvailable
refresh_rect.Union(element.GetRedrawRect())
File "/home/developer/WorkData/PLC/beremiz/beremiz/graphics/GraphicCommons.py", line 1609, in GetRedrawRect
self.ValueSize = self.Parent.GetMiniTextExtent(self.ComputedValue)
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 932, in GetMiniTextExtent
return self.MiniTextDC.GetTextExtent(text)
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk3/wx/_gdi.py", line 4127, in GetTextExtent
return _gdi_.DC_GetTextExtent(*args, **kwargs)
<class 'wx._core.PyAssertionError'>: C++ assertion "m_graphicContext" failed at ../src/common/dcgraph.cpp(1160) in DoGetTextExtent(): wxGCDC(cg)::DoGetTextExtent - invalid DC

wx-platform:
__WXGTK__

wx-version:
3.0.2.0

[-------------------------------------------------------------------------------------------------------------------]

traceback:
File "/home/developer/WorkData/PLC/beremiz/beremiz/controls/LogViewer.py", line 740, in OnMessageToolTipTimer
self.MessageToolTip.SetFont(self.Font)
File "/home/developer/WorkData/PLC/beremiz/beremiz/controls/CustomToolTip.py", line 75, in SetFont
self.RefreshTip()
File "/home/developer/WorkData/PLC/beremiz/beremiz/controls/CustomToolTip.py", line 158, in RefreshTip
self.SetClientSize(self.GetToolTipSize())
File "/home/developer/WorkData/PLC/beremiz/beremiz/controls/CustomToolTip.py", line 145, in GetToolTipSize
w, h = dc.GetTextExtent(line)
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk3/wx/_gdi.py", line 4127, in GetTextExtent
return _gdi_.DC_GetTextExtent(*args, **kwargs)
<class 'wx._core.PyAssertionError'>: C++ assertion "m_graphicContext" failed at ../src/common/dcgraph.cpp(1160) in DoGetTextExtent(): wxGCDC(cg)::DoGetTextExtent - invalid DC

[-------------------------------------------------------------------------------------------------------------------]

# SOME DESCRIPTIVE TITLE.
# Copyright (C) is 21st century inquisition
# This file is distributed under the same license as the Beremiz package.
# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
#
msgid ""
msgstr ""
"Project-Id-Version: Beremiz 1.1\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2012-09-08 00:41\n"
"PO-Revision-Date: 2012-09-07 10:53+0100\n"
"Last-Translator: \n"
"Language-Team: \n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"X-Poedit-Language: French\n"
"X-Poedit-Country: FRANCE\n"
"X-Poedit-SourceCharset: utf-8\n"

# c9c5ea5d04e64cb6a54fcda391cb4929
#: ../../manual/install.rst:2
msgid "Beremiz installation"
msgstr "Installation de Beremiz"

# 3f78e50eff364a0abbfcb8cf9a0a68da
#: ../../manual/install.rst:5
msgid "Windows"
msgstr "WIndows"

# 963db7a095b246d68c4a51163b441952
#: ../../manual/install.rst:6
msgid "Download installer, install."
msgstr "Téléchergez l'installeur, exécutez le."

# 87380693201647979e3c0d0527205575
#: ../../manual/install.rst:9
msgid "Linux"
msgstr "Linux"

# 0c4d6e88a20248b9bc3b167cf120c318
#: ../../manual/install.rst:10
msgid "Pre-requisites::"
msgstr "Pre-requis::"

# ced14d340a194a2b9f53b8e6c34d6804
#: ../../manual/install.rst:16
msgid "Prepare::"
msgstr "Préparation::"

# f555cb10ef0d42d6bbbe2f85f0763057
#: ../../manual/install.rst:21
msgid "Get Source Code::"
msgstr "Obtenir le code source::"

# 1d5926626d3b49d891529b9fc687fec3
#: ../../manual/install.rst:29
msgid "Build MatIEC compiler::"
msgstr "Compiler le compilateur MatIEC::"

# 166772d03f6f4d67abb4ed849fc3f6d8
#: ../../manual/install.rst:35
msgid "Build CanFestival (optional)::"
msgstr "Compiler CanFestival::"

# 39a65c2b7ca747438a15d31bd4a40a4f
#: ../../manual/install.rst:47
msgid "Launch Beremiz::"
msgstr "Lancer Beremiz::"