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

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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

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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.

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# Copyright (C) is 21st century inquisition

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# This file is distributed under the same license as the Beremiz package.

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# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.

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#

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msgid ""

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msgstr ""

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"Project-Id-Version: Beremiz 1.1\n"

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"Report-Msgid-Bugs-To: \n"

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"POT-Creation-Date: 2012-09-08 00:39\n"

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"PO-Revision-Date: 2012-09-07 10:48+0100\n"

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"Last-Translator: \n"

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"Language-Team: \n"

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"Language: \n"

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"MIME-Version: 1.0\n"

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"Content-Type: text/plain; charset=UTF-8\n"

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"Content-Transfer-Encoding: 8bit\n"

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"X-Poedit-Language: French\n"

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"X-Poedit-Country: FRANCE\n"

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"X-Poedit-SourceCharset: utf-8\n"

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# e69e0ee137df409290c6c6a5fbd512a6

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#: ../../index.rst:4

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msgid "Beremiz's documentation"

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msgstr "Documentation de Beremiz"

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# 7a5a9d62913b477b99421d5dc94a44be

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#: ../../index.rst:6

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msgid "Contents:"

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msgstr "Contenu"

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# 899ecb5f4ff84b8e98dbf5b1eaf7f393

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#~ msgid "Indices and tables"

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#~ msgstr "Index et tables"