PcdbNew [nightly] Assertion failed on RMB

[Michele Giorato]

More and more problems are appearing with the drop-down menus that appear when the right mouse button is pressed.

ASSERT INFO:
../src/gtk/bitmap.cpp(917): assert "IsOk()" failed in GetHeight(): invalid bitmap

BACKTRACE:
[1] wxBitmap::GetHeight() const
[2] make_fcontext

In the main menu the View item does not always appear, sometimes it has scroll buttons, sometimes it is correct, sometimes it closes everything (not only pcbnew).
Sometimes even Place and Edit are empty and don't even react by clicking on it.

Application: Pcbnew
Version: 6.0.0-unknown-r15850-b5c80fe6, debug build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.64.0 OpenSSL/1.1.1b zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.20.2 (+libidn2/2.0.5) libssh/0.8.7/openssl/zlib nghttp2/1.38.0
Platform: Linux 5.0.17-300.fc30.x86_64 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: 1.69.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.64.0
    Compiler: GCC 9.1.1 with C++ ABI 1013

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