Eeschema standalone crashes importing Eagle on Bus Connection Test (lp:#1831608)

[Original report](https://bugs.launchpad.net/bugs/1831608) created by **Seth Hillbrand (sethh)**
```
Step to reproduce:
- Open Eeeschema
- Import Non-KiCad sheet
- Select the attached file

#0  0x00007fffef170e56 in SCH_CONNECTION::IsBus() const (this=0x0) at
../../eeschema/./sch_connection.h:125
#1  0x00007fffef1a8872 in CONNECTION_GRAPH::matchBusMember(SCH_CONNECTION*,
SCH_CONNECTION*) (aBusConnection=0x0, aSearch=0x55555c441190)
    at ../../eeschema/connection_graph.cpp:1653
#2  0x00007fffef1a5c53 in CONNECTION_GRAPH::buildConnectionGraph()
(this=0x555556e99cb0) at ../../eeschema/connection_graph.cpp:1343
#3  0x00007fffef1a0da6 in CONNECTION_GRAPH::Recalculate(SCH_SHEET_LIST,
bool) (this=0x555556e99cb0, aSheetList=..., aUnconditional=true)
    at ../../eeschema/connection_graph.cpp:417
#4  0x00007fffef2fe148 in SCH_EDIT_FRAME::RecalculateConnections(bool)
(this=0x55555692ef90, aDoCleanup=true) at
../../eeschema/sch_edit_frame.cpp:1139
#5  0x00007fffef1f2d71 in SCH_EDIT_FRAME::importFile(wxString const&, int)
(this=0x55555692ef90, aFileName=..., aFileType=1)
    at ../../eeschema/files-io.cpp:868
#6  0x00007fffef1f1998 in SCH_EDIT_FRAME::OnImportProject(wxCommandEvent&)
(this=0x55555692ef90, aEvent=...) at ../../eeschema/files-io.cpp:697
#7  0x00007ffff74b67ae in
wxEvtHandler::ProcessEventIfMatchesId(wxEventTableEntryBase const&,
wxEvtHandler*, wxEvent&) ()
    at /usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#8  0x00007ffff74b68b3 in wxEventHashTable::HandleEvent(wxEvent&,
wxEvtHandler*) () at /usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#9  0x00007ffff74b6bec in wxEvtHandler::TryHereOnly(wxEvent&) () at
/usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#10 0x00007fffef487f7f in EDA_BASE_FRAME::ProcessEvent(wxEvent&)
(this=0x55555692ef90, aEvent=...) at ../../common/eda_base_frame.cpp:183
#11 0x00007ffff74b6a73 in wxEvtHandler::DoTryChain(wxEvent&) () at
/usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#12 0x00007ffff74b6d11 in wxEvtHandler::ProcessEvent(wxEvent&) () at
/usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#13 0x00007ffff7a1840b in wxWindowBase::TryAfter(wxEvent&) () at
/usr/lib/x86_64-linux-gnu/libwx_gtk3u_core-3.0.so.0
#14 0x00007ffff74b6ab7 in wxEvtHandler::SafelyProcessEvent(wxEvent&) () at
/usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#15 0x00007ffff79cf9e5 in wxMenuBase::SendEvent(int, int) () at
/usr/lib/x86_64-linux-gnu/libwx_gtk3u_core-3.0.so.0
#16 0x00007ffff78cf10b in  () at /usr/lib/x86_64-linux-
gnu/libwx_gtk3u_core-3.0.so.0
#17 0x00007ffff5a97c7d in g_closure_invoke () at /usr/lib/x86_64-linux-
gnu/libgobject-2.0.so.0
#18 0x00007ffff5aab345 in  () at /usr/lib/x86_64-linux-
gnu/libgobject-2.0.so.0
#19 0x00007ffff5ab425e in g_signal_emit_valist () at /usr/lib/x86_64-linux-
gnu/libgobject-2.0.so.0
#20 0x00007ffff5ab491f in g_signal_emit () at /usr/lib/x86_64-linux-
gnu/libgobject-2.0.so.0
#21 0x00007ffff5f5588a in gtk_widget_activate () at /usr/lib/x86_64-linux-
gnu/libgtk-3.so.0
#22 0x00007ffff5e28a86 in gtk_menu_shell_activate_item () at
/usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#23 0x00007ffff5e28d23 in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#24 0x00007ffff5fa5274 in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#25 0x00007ffff5a97eb6 in  () at /usr/lib/x86_64-linux-
gnu/libgobject-2.0.so.0
#26 0x00007ffff5ab3d14 in g_signal_emit_valist () at /usr/lib/x86_64-linux-
gnu/libgobject-2.0.so.0
#27 0x00007ffff5ab491f in g_signal_emit () at /usr/lib/x86_64-linux-
gnu/libgobject-2.0.so.0
#28 0x00007ffff5f53324 in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#29 0x00007ffff5e13976 in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#30 0x00007ffff5e15a83 in gtk_main_do_event () at /usr/lib/x86_64-linux-
gnu/libgtk-3.so.0
#31 0x00007ffff5b15465 in  () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#32 0x00007ffff5b46112 in  () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#33 0x00007ffff5803f2e in g_main_context_dispatch () at
/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#34 0x00007ffff58041c8 in  () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#35 0x00007ffff58044c2 in g_main_loop_run () at /usr/lib/x86_64-linux-
gnu/libglib-2.0.so.0
#36 0x00007ffff5e14b15 in gtk_main () at /usr/lib/x86_64-linux-
gnu/libgtk-3.so.0
#37 0x00007ffff785b1d5 in wxGUIEventLoop::DoRun() () at
/usr/lib/x86_64-linux-gnu/libwx_gtk3u_core-3.0.so.0
#38 0x00007ffff738c48d in wxEventLoopBase::Run() () at
/usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#39 0x00007ffff7355616 in wxAppConsoleBase::MainLoop() () at
/usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#40 0x00005555555972af in APP_SINGLE_TOP::OnRun() (this=0x5555556aa130) at
../../common/single_top.cpp:197
#41 0x00007ffff73d6cf9 in wxEntry(int&, wchar_t**) () at
/usr/lib/x86_64-linux-gnu/libwx_baseu-3.0.so.0
#42 0x0000555555592db6 in main(int, char**) (argc=1, argv=0x7fffffffe118)
at ../../common/single_top.cpp:271


Application: KiCad
Version: (5.1.0-846-ga5f670b69-dirty), debug build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.64.0 OpenSSL/1.1.1b zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2
(+libidn2/2.0.5) libssh2/1.8.0 nghttp2/1.36.0 librtmp/2.3
Platform: Linux 4.19.0-5-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: 7.64.0
    Compiler: GCC 8.3.0 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
```
Original tags: eagle eeschema

Story points: 6.0