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

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

<?xml version="1.0"?>
<!-- generated by wxGlade 0.7.2 on Mon Jun  5 14:18:29 2017 -->

<application class="" encoding="UTF-8" for_version="2.8" header_extension=".h" indent_amount="4" indent_symbol="space" is_template="0" language="python" name="" option="0" overwrite="0" path="..py.py" source_extension=".cpp" top_window="HMIFrame" use_gettext="0" use_new_namespace="1">
    <object class="Class_HMIFrame" name="HMIFrame" base="EditFrame">
        <style>wxDEFAULT_FRAME_STYLE</style>
        <title>HMIFrame</title>
        <object class="wxBoxSizer" name="sizer_1" base="EditBoxSizer">
            <orient>wxVERTICAL</orient>
            <object class="sizeritem">
                <border>0</border>
                <option>0</option>
                <object class="wxSpinCtrl" name="spin_ctrl_1" base="EditSpinCtrl">
                    <range>0, 10000</range>
                </object>
            </object>
            <object class="sizeritem">
                <border>0</border>
                <option>0</option>
                <object class="wxCheckBox" name="checkbox_1" base="EditCheckBox">
                    <label>checkbox_1</label>
                </object>
            </object>
            <object class="sizeritem">
                <border>0</border>
                <option>0</option>
                <object class="wxStaticText" name="label_1" base="EditStaticText">
                    <attribute>1</attribute>
                    <label>GUI changed!</label>
                </object>
            </object>
        </object>
    </object>
</application>