fix backtrace on opening program/resource on small screens

Fix regression introduced by f4696d8 ("add stretching to columns Name,
Type, Doc in variable panel").
This problem happens only on wxPython-3.0.

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                  <xsd:element name="Win32">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                    </xsd:complexType>

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                  </xsd:element>

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