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

# Copyright (C) 2018: Smarteh
#
# See COPYING file for copyrights details.


from __future__ import absolute_import
from __future__ import print_function

import wx
from zope.interface import implementer

from controls.UriLocationEditor import IConnectorPanel

URITypes = ["LOCAL", "PYRO", "PYROS"]


def PYRO_connector_dialog(confnodesroot):
    [ID_IPTEXT, ID_PORTTEXT] = [wx.NewId() for _init_ctrls in range(2)]

    @implementer(IConnectorPanel)
    class PYROConnectorPanel(wx.Panel):
        def __init__(self, typeConnector, parrent, *args, **kwargs):
            self.type = typeConnector
            self.parrent = parrent
            wx.Panel.__init__(self, parrent, *args, **kwargs)
            self._init_ctrls()
            self._init_sizers()
            self.uri = None

        def _init_ctrls(self):
            self.IpText = wx.TextCtrl(parent=self, id=ID_IPTEXT, size=wx.Size(200, -1))
            self.PortText = wx.TextCtrl(parent=self, id=ID_PORTTEXT, size=wx.Size(200, -1))

        def _init_sizers(self):
            self.mainSizer = wx.FlexGridSizer(cols=2, hgap=10, rows=5, vgap=10)
            self.mainSizer.AddWindow(wx.StaticText(self, label=_("URI host:")),
                                     flag=wx.ALIGN_CENTER_VERTICAL)
            self.mainSizer.AddWindow(self.IpText, flag=wx.GROW)

            self.mainSizer.AddWindow(wx.StaticText(self, label=_("URI port:")),
                                     flag=wx.ALIGN_CENTER_VERTICAL)
            self.mainSizer.AddWindow(self.PortText, flag=wx.GROW)
            self.SetSizer(self.mainSizer)

        def SetURI(self, uri):
            self.uri = uri
            uri_list = uri.strip().split(":")
            length = len(uri_list)
            if length == 3:
                self.IpText.SetValue(uri_list[1].strip("/"))
                self.PortText.SetValue(uri_list[2])
            elif length == 2:
                self.IpText.SetValue(uri_list[1].strip("/"))

        def GetURI(self):
            self.uri = self.type+"://"+self.IpText.GetValue()+":"+self.PortText.GetValue()
            return self.uri

    return PYROConnectorPanel("PYRO", confnodesroot)