Build Information:
TShark (Wireshark) 2.6.3 (Git v2.6.3 packaged as 2.6.3-1~ubuntu14.04.1)

Copyright 1998-2018 Gerald Combs <email address hidden> and contributors.
License GPLv2+: GNU GPL version 2 or later <http://www.gnu.org/licenses/old-licenses/gpl-2.0.html>
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Compiled (64-bit) with libpcap, with POSIX capabilities (Linux), with libnl 3,
with GLib 2.40.2, with zlib 1.2.8, with SMI 0.4.8, with c-ares 1.10.0, with Lua
5.2.3, with GnuTLS 2.12.23, with Gcrypt 1.5.3, with MIT Kerberos, without
MaxMind DB resolver, without nghttp2, with LZ4, with Snappy, with libxml2 2.9.1.

Running on Linux 4.4.0-31-generic, with Intel Core Processor (Skylake, IBRS)
(with SSE4.2), with 2000 MB of physical memory, with locale
LC_CTYPE=en_US.UTF-8, LC_NUMERIC=hu_HU.UTF-8, LC_TIME=hu_HU.UTF-8,
LC_COLLATE=en_US.UTF-8, LC_MONETARY=hu_HU.UTF-8, LC_MESSAGES=en_US.UTF-8,
LC_PAPER=hu_HU.UTF-8, LC_NAME=hu_HU.UTF-8, LC_ADDRESS=hu_HU.UTF-8,
LC_TELEPHONE=hu_HU.UTF-8, LC_MEASUREMENT=hu_HU.UTF-8,
LC_IDENTIFICATION=hu_HU.UTF-8, with libpcap version 1.5.3, with GnuTLS 2.12.23,
with Gcrypt 1.5.3, with zlib 1.2.8, binary plugins supported (13 loaded).

Built using gcc 4.8.4.
--
Reproduction:

1. Start the Qt GUI
2. Close the application using the window's close icon on the title bar (x).

The following crash can be observed when doing so with Valgrind:

==3163== Invalid read of size 8
==3163==    at 0xC0FF338: QIcon::~QIcon() (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC108BD7: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC1052D8: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC1053D8: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC0FF33A: QIcon::~QIcon() (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC0FF378: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xDBE71A8: __run_exit_handlers (exit.c:82)
==3163==    by 0xDBE71F4: exit (exit.c:104)
==3163==    by 0x275752: exit_application (in /usr/bin/wireshark)
==3163==    by 0x2712B7: main (in /usr/bin/wireshark)
==3163==  Address 0x32cdad38 is not stack'd, malloc'd or (recently) free'd
==3163== 
==3163== 
==3163== Process terminating with default action of signal 11 (SIGSEGV)
==3163==  Access not within mapped region at address 0x32CDAD38
==3163==    at 0xC0FF338: QIcon::~QIcon() (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC108BD7: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC1052D8: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC1053D8: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC0FF33A: QIcon::~QIcon() (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xC0FF378: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==3163==    by 0xDBE71A8: __run_exit_handlers (exit.c:82)
==3163==    by 0xDBE71F4: exit (exit.c:104)
==3163==    by 0x275752: exit_application (in /usr/bin/wireshark)
==3163==    by 0x2712B7: main (in /usr/bin/wireshark)
==3163==  If you believe this happened as a result of a stack
==3163==  overflow in your program's main thread (unlikely but
==3163==  possible), you can try to increase the size of the
==3163==  main thread stack using the --main-stacksize= flag.
==3163==  The main thread stack size used in this run was 8388608.
==3163== 
==3163== HEAP SUMMARY:
==3163==     in use at exit: 14,502,711 bytes in 33,680 blocks
==3163==   total heap usage: 734,425 allocs, 700,745 frees, 257,404,770 bytes allocated
==3163== 
==3163== LEAK SUMMARY:
==3163==    definitely lost: 6,916 bytes in 75 blocks
==3163==    indirectly lost: 23,488 bytes in 640 blocks
==3163==      possibly lost: 55,100 bytes in 806 blocks
==3163==    still reachable: 14,125,663 bytes in 31,106 blocks
==3163==         suppressed: 0 bytes in 0 blocks
==3163== Rerun with --leak-check=full to see details of leaked memory
==3163== 
==3163== For counts of detected and suppressed errors, rerun with: -v
==3163== Use --track-origins=yes to see where uninitialised values come from
==3163== ERROR SUMMARY: 8 errors from 8 contexts (suppressed: 2 from 1)

It seems to be the same problem as reported here:
https://bugreports.qt.io/browse/QTBUG-27005?page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel&showAll=true

I can also reproduce the issue on Ubuntu 16.04 with valgrind, but not on Ubuntu 18.04.