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

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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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--- a/controls/CustomToolTip.py	Tue Jun 26 14:48:44 2018 +0300
+++ b/controls/CustomToolTip.py	Tue Jun 26 17:34:15 2018 +0300
@@ -137,7 +137,7 @@
         max_width = max_height = 0
 
         # Create a memory DC for calculating text extent
-        dc = wx.MemoryDC()
+        dc = wx.MemoryDC(wx.EmptyBitmap(1, 1))
         dc.SetFont(self.Font)
 
         # Compute max tip text size

--- a/editors/Viewer.py	Tue Jun 26 14:48:44 2018 +0300
+++ b/editors/Viewer.py	Tue Jun 26 17:34:15 2018 +0300
@@ -770,7 +770,7 @@
             dc.SetFont(font)
             width, _height = dc.GetTextExtent("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
         self.SetFont(font)
-        self.MiniTextDC = wx.MemoryDC()
+        self.MiniTextDC = wx.MemoryDC(wx.EmptyBitmap(1, 1))
         self.MiniTextDC.SetFont(wx.Font(faces["size"] * 0.75, wx.SWISS, wx.NORMAL, wx.NORMAL, faceName=faces["helv"]))
 
         self.CurrentScale = None