Paste crashes the symbol editor

1. Open the symbol editor so that the (system) clipboard is empty.
2. Open a symbol.
3. Context menu->Paste.

KiCad crashes.

There's still the problem which I have reported before that Paste menu item is active even when there's nothing to paste. I think both the clipboard content and the context in which the menu is opened should always be checked to see if the Paste item should be active or not. That would prevent almost all possible crashes depending on incompatible clipboard content.

Application: KiCad
Version: (5.1.0-824-g15e678759), release build
Libraries:
    wxWidgets 3.0.4
Platform: Linux 4.15.0-50-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.22
    Boost: 1.65.1
    OpenCASCADE Community Edition: 6.9.1
    Compiler: GCC 7.3.0 with C++ ABI 1011

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=OFF
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=OFF