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

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

[FileInfo]
FileName=slave.eds
FileVersion=1
FileRevision=1
EDSVersion=4.0
Description=
CreationTime=09:10AM
CreationDate=10-27-2008
CreatedBy=CANFestival
ModificationTime=09:10AM
ModificationDate=10-27-2008
ModifiedBy=CANFestival

[DeviceInfo]
VendorName=CANFestival
VendorNumber=0x00000000
ProductName=SlaveNode
ProductNumber=0x00000000
RevisionNumber=0x00000000
BaudRate_10=1
BaudRate_20=1
BaudRate_50=1
BaudRate_125=1
BaudRate_250=1
BaudRate_500=1
BaudRate_800=1
BaudRate_1000=1
SimpleBootUpMaster=0
SimpleBootUpSlave=1
Granularity=8
DynamicChannelsSupported=0
CompactPDO=0
GroupMessaging=0
NrOfRXPDO=4
NrOfTXPDO=4
LSS_Supported=0

[DummyUsage]
Dummy0001=0
Dummy0002=1
Dummy0003=1
Dummy0004=1
Dummy0005=1
Dummy0006=1
Dummy0007=1

[Comments]
Lines=0

[MandatoryObjects]
SupportedObjects=3
1=0x1000
2=0x1001
3=0x1018

[1000]
ParameterName=Device Type
ObjectType=0x7
DataType=0x0007
AccessType=ro
DefaultValue=0
PDOMapping=0

[1001]
ParameterName=Error Register
ObjectType=0x7
DataType=0x0005
AccessType=ro
DefaultValue=0
PDOMapping=1

[1018]
ParameterName=Identity
ObjectType=0x8
SubNumber=5

[1018sub0]
ParameterName=Number of Entries
ObjectType=0x7
DataType=0x0005
AccessType=ro
DefaultValue=4
PDOMapping=0

[1018sub1]
ParameterName=Vendor ID
ObjectType=0x7
DataType=0x0007
AccessType=ro
DefaultValue=0
PDOMapping=0

[1018sub2]
ParameterName=Product Code
ObjectType=0x7
DataType=0x0007
AccessType=ro
DefaultValue=0
PDOMapping=0

[1018sub3]
ParameterName=Revision Number
ObjectType=0x7
DataType=0x0007
AccessType=ro
DefaultValue=0
PDOMapping=0

[1018sub4]
ParameterName=Serial Number
ObjectType=0x7
DataType=0x0007
AccessType=ro
DefaultValue=0
PDOMapping=0

[OptionalObjects]
SupportedObjects=1
1=0x1200

[1200]
ParameterName=Server SDO Parameter
ObjectType=0x8
SubNumber=3

[1200sub0]
ParameterName=Number of Entries
ObjectType=0x7
DataType=0x0005
AccessType=ro
DefaultValue=2
PDOMapping=0

[1200sub1]
ParameterName=COB ID Client to Server (Receive SDO)
ObjectType=0x7
DataType=0x0007
AccessType=ro
DefaultValue=$NODEID+0x600
PDOMapping=0

[1200sub2]
ParameterName=COB ID Server to Client (Transmit SDO)
ObjectType=0x7
DataType=0x0007
AccessType=ro
DefaultValue=$NODEID+0x580
PDOMapping=0

[ManufacturerObjects]
SupportedObjects=2
1=0x2000
2=0x2001

[2000]
ParameterName=OutVar
ObjectType=0x7
DataType=0x0003
AccessType=rw
DefaultValue=0
PDOMapping=1

[2001]
ParameterName=InVar
ObjectType=0x7
DataType=0x0003
AccessType=rw
DefaultValue=0
PDOMapping=1