Segfault while editing net classes

[Fabián Inostroza]

- Open any project and go to board setup->net classes
- Click on any of the existing nets, specifically in the "Net class" column
- Create a new net class
- Click again on any of the existing nets
- Seg fault

Application: kicad
Version: (6.0.0-rc1-dev-1426-gdde933ba0-dirty), release build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.62.0 GnuTLS/3.5.19 zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2 (+libidn2/2.0.5) libssh2/1.8.0 nghttp2/1.35.1 librtmp/2.3
Platform: Linux 4.18.0-3-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
    Boost: 1.67.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.62.0
    Compiler: GCC 8.2.0 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=ON
    KICAD_SCRIPTING_WXPYTHON=OFF
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=OFF