Tor Browser 7 crashes when cancelling downloads on the external helper app warning

Tor Browser 7.5a1 (7.0.1 doesn't have this issue as far as I tested)

Platform: Debian sid 64 bits.

Steps to reproduce:

1. Open Tor Browser,
2. Ctrl+O
3. Select some PDF file and click open
4. Download box will show up, click on cancel
5. it should crash

When debugging:

[Parent 3674] ###!!! ABORT: __delete__()d actor: file /home/debian/build/tor-browser/ipc/glue/ProtocolUtils.cpp, line 299
[Parent 3674] ###!!! ABORT: __delete__()d actor: file /home/debian/build/tor-browser/ipc/glue/ProtocolUtils.cpp, line 299
Crash Annotation GraphicsCriticalError: |[C0][GFX1-]: Receive IPC close with reason=AbnormalShutdown (t=8.69987) [GFX1-]: Receive IPC close with reason=AbnormalShutdown
[Child 3795] WARNING: pipe error (3): Connection reset by peer: file /home/debian/build/tor-browser/ipc/chromium/src/chrome/common/ipc_channel_posix.cc, line 322
[Child 3795] ###!!! ABORT: Aborting on channel error.: file /home/debian/build/tor-browser/ipc/glue/MessageChannel.cpp, line 2152
[Child 3795] ###!!! ABORT: Aborting on channel error.: file /home/debian/build/tor-browser/ipc/glue/MessageChannel.cpp, line 2152
[notice] Owning controller connection has closed -- exiting now.
[notice] Catching signal TERM, exiting cleanly.
./Browser/start-tor-browser: line 369:  3674 Segmentation fault      (core dumped) TOR_CONTROL_PASSWD=${TOR_CONTROL_PASSWD} ./firefox --class "Tor Browser" -profile TorBrowser/Data/Browser/profile.default "${@}" < /dev/null