Here is the backtrace:

Thread 1 "indicator-datet" received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50	../sysdeps/unix/sysv/linux/raise.c: Datei oder Verzeichnis nicht gefunden.
(gdb) bt
#0  0x00007ffff6eaa3eb in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007ffff6e89899 in __GI_abort () at abort.c:79
#2  0x00007ffff7283b23 in g_assertion_message
    (domain=<optimized out>, file=<optimized out>, line=<optimized out>, func=0x5555556310c8 "GDateTime* unity::indicator::datetime::DateTime::get() const", message=<optimized out>)
    at ../../../glib/gtestutils.c:2912
#3  0x00007ffff72e032f in g_assertion_message_expr
    (domain=domain@entry=0x55555562d017 "Indicator-Datetime", file=file@entry=0x555555631108 "/build/indicator-datetime-P60bPZ/indicator-datetime-15.10+19.10.20190819.1/src/date-time.cpp", line=line@entry=171, func=func@entry=0x5555556310c8 "GDateTime* unity::indicator::datetime::DateTime::get() const", expr=expr@entry=0x5555556310a0 "m_dt") at ../../../glib/gtestutils.c:2938
#4  0x00005555555cda9c in unity::indicator::datetime::DateTime::get() const (this=0x7fffffffd878) at ./src/date-time.cpp:171
#5  0x00005555555cdd15 in unity::indicator::datetime::DateTime::get() const (this=0x7fffffffd878) at ./src/date-time.cpp:171
#6  0x00005555555cdd15 in unity::indicator::datetime::DateTime::format(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const
    (this=this@entry=0x7fffffffd878, fmt="%F %T %z") at ./src/date-time.cpp:179
#7  0x00005555555d2d83 in unity::indicator::datetime::EdsEngine::Impl::get_appointment(_ECalClient*, std::shared_ptr<_GCancellable>&, _ECalComponent*, _GTimeZone*) (client=<optimized out>, cancellable=Warnung: RTTI symbol not found for class 'std::_Sp_counted_deleter<_GCancellable*, unity::indicator::datetime::EdsEngine::Impl::Impl(std::shared_ptr<unity::indicator::datetime::Myself> const&)::{lambda(_GCancellable*)#1}, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>'
Warnung: RTTI symbol not found for class 'std::_Sp_counted_deleter<_GCancellable*, unity::indicator::datetime::EdsEngine::Impl::Impl(std::shared_ptr<unity::indicator::datetime::Myself> const&)::{lambda(_GCancellable*)#1}, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>'

std::shared_ptr<struct _GCancellable> (use count 23, weak count 0) = {...}, component=component@entry=0x7fffe405c560 [ECalComponent], gtz=gtz@entry=0x5555556f1960)
    at /usr/include/c++/9/bits/char_traits.h:300
#8  0x00005555555dfe7e in unity::indicator::datetime::EdsEngine::Impl::add_event_to_subtask(_ECalComponent*, unity::indicator::datetime::EdsEngine::Impl::ClientSubtask*, _GTimeZone*)
    (gtz=0x5555556f1960, subtask=0x555555774a50, component=0x7fffe405c560 [ECalComponent]) at ./src/engine-eds.cpp:1166
#9  0x00005555555dfe7e in unity::indicator::datetime::EdsEngine::Impl::on_event_fetch_list_done(void*) (gsubtask=0x555555774a50) at ./src/engine-eds.cpp:866
#10 0x00005555555dfe7e in unity::indicator::datetime::EdsEngine::Impl::fetch_detached_instances(_GObject*, _GAsyncResult*, void*) (res=<optimized out>, gsubtask=0x555555774a50)
    at ./src/engine-eds.cpp:744
#11 0x00007ffff7d2fcc5 in free_get_objects_async_data (goad=0x555555788b90) at ./src/calendar/libecal/e-cal-client.c:2587
#12 0x00007ffff7d371e4 in got_object_list_as_comps_cb (source_object=<optimized out>, result=0x5555557a1320, user_data=user_data@entry=0x555555788b90) at ./src/calendar/libecal/e-cal-client.c:2664
#13 0x00007ffff748acf6 in g_simple_async_result_complete (simple=0x5555557a1320 [GSimpleAsyncResult]) at ../../../gio/gsimpleasyncresult.c:802
#14 0x00007ffff748adbf in complete_in_idle_cb_for_thread (_data=0x5555557a1a40) at ../../../gio/gsimpleasyncresult.c:873
#15 0x00007ffff72b771e in g_main_dispatch (context=0x5555556a4b80) at ../../../glib/gmain.c:3179
#16 0x00007ffff72b771e in g_main_context_dispatch (context=context@entry=0x5555556a4b80) at ../../../glib/gmain.c:3844
#17 0x00007ffff72b7ad0 in g_main_context_iterate (context=0x5555556a4b80, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../../../glib/gmain.c:3917
#18 0x00007ffff72b7dc3 in g_main_loop_run (loop=0x5555556e1fe0) at ../../../glib/gmain.c:4111
#19 0x00005555555b2d55 in main(int, char**) () at ./src/main.cpp:183