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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#!/usr/bin/env python

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# -*- coding: utf-8 -*-

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# This file is part of Beremiz, a Integrated Development Environment for

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# programming IEC 61131-3 automates supporting plcopen standard and CanFestival.

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#

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# Copyright (C) 2007: Edouard TISSERANT and Laurent BESSARD

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#

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# See COPYING file for copyrights details.

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#

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# This program is free software; you can redistribute it and/or

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# modify it under the terms of the GNU General Public License

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# as published by the Free Software Foundation; either version 2

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# of the License, or (at your option) any later version.

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#

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# This program is distributed in the hope that it will be useful,

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# but WITHOUT ANY WARRANTY; without even the implied warranty of

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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

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# GNU General Public License for more details.

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#

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# You should have received a copy of the GNU General Public License

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# along with this program; if not, write to the Free Software

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# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.

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from __future__ import absolute_import

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import wx

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from plcopen.structures import TestIdentifier, IEC_KEYWORDS

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# -------------------------------------------------------------------------------

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#                                POU Name Dialog

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# -------------------------------------------------------------------------------

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class PouNameDialog(wx.TextEntryDialog):

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    def __init__(self, parent, message, caption=_("Please enter text"), defaultValue="",

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                 style=wx.OK | wx.CANCEL | wx.CENTRE, pos=wx.DefaultPosition):

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        wx.TextEntryDialog.__init__(self, parent, message, caption, defaultValue, style, pos)

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        self.PouNames = []

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        self.Bind(wx.EVT_BUTTON, self.OnOK,

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                  self.GetSizer().GetItem(2).GetSizer().GetItem(1).GetSizer().GetAffirmativeButton())

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    def OnOK(self, event):

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        message = None

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        step_name = self.GetSizer().GetItem(1).GetWindow().GetValue()

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        if step_name == "":

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            message = _("You must type a name!")

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        elif not TestIdentifier(step_name):

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            message = _("\"%s\" is not a valid identifier!") % step_name

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        elif step_name.upper() in IEC_KEYWORDS:

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            message = _("\"%s\" is a keyword. It can't be used!") % step_name

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        elif step_name.upper() in self.PouNames:

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            message = _("A POU named \"%s\" already exists!") % step_name

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        if message is not None:

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            dialog = wx.MessageDialog(self, message, _("Error"), wx.OK | wx.ICON_ERROR)

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            dialog.ShowModal()

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            dialog.Destroy()

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        else:

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            self.EndModal(wx.ID_OK)

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        event.Skip()

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    def SetPouNames(self, pou_names):

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        self.PouNames = [pou_name.upper() for pou_name in pou_names]