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

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# 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.
#
#, fuzzy
msgid ""
msgstr ""
"Project-Id-Version: Beremiz 1.1\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2012-09-08 01:47\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <LL@li.org>\n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"

# 175ab223adbc42749a504a6f9718fb71
#: ../../standards.rst:2
msgid "IEC 61131-3"
msgstr ""

# c2d98f5b3a4c4f6fa6fdab3b7d419f28
#: ../../standards.rst:4
msgid ""
"IEC-61131 is a normative document provided by the standards organization IEC "
"(International Electrotechnical Commission) and describing a standard for "
"implementing programmable controllers."
msgstr ""

# da7a80107e094184af3a7d6504a3718b
#: ../../standards.rst:9
msgid ""
"The part 3 of this document (commonly named IEC 61131-3) specifies syntax "
"and semantics for programming language for programmable controllers. Beremiz "
"implements all the languages described in this document."
msgstr ""

# 98082fb5f0564eb38341fd5865f3cef6
#: ../../standards.rst:14
msgid "http://www.iec.eu"
msgstr ""

# fb0bef609e7b43079133dc137afbdc34
#: ../../standards.rst:17
msgid "PLCopen TC6"
msgstr ""

# abf4b2a73aa54caba919975385b1b2af
#: ../../standards.rst:19
msgid ""
"PLCopen is a vendor- and product-independent worldwide association defining "
"international standards for various topics related to control programming. "
"For this purpose, PLCopen has 6 technical committees."
msgstr ""

# aa4df8a9f69749cdb44143a2f6a973e2
#: ../../standards.rst:24
msgid ""
"The goal of the sixth committee (TC6) is to define a standard file format, "
"based on XML, for exchanging programmables controllers programmed using IEC "
"61131-3 languages. Beremiz uses this file format for saving the PLC programs "
"of projects."
msgstr ""

# 3c0fd475a8374f8d843d142081bdaf35
#: ../../standards.rst:30
msgid "http://www.plcopen.org"
msgstr ""