Pinta suddenly exits with an error after a few seconds

Steps to reproduce:
1. Install Pinta using ubuntu-software
2. Open Pinta
3. Do anything at all for a few seconds (I opened an image in order to draw a line on it)
4. After a few seconds Pinta will disappear without any notice

When I run Pinta from the command line it exits with this error:
```
➜ ~ pinta
Gtk-Message: 15:05:45.554: Failed to load module "canberra-gtk-module"
free(): invalid pointer
Stacktrace:

  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) GLib.SList.g_free (intptr) <0x0005f>
  at GLib.ListBase.Empty () <0x0013c>
  at GLib.ListBase.Dispose (bool) <0x0000f>
  at GLib.ListBase.Finalize () <0x0001d>
  at (wrapper runtime-invoke) object.runtime_invoke_virtual_void__this__ (object,intptr,intptr,intptr) <0x00068>

Native stacktrace:

        /usr/bin/mono(+0xc8514) [0x55cdb4295514]
        /lib/x86_64-linux-gnu/libpthread.so.0(+0x12890) [0x7fa64485a890]
        /lib/x86_64-linux-gnu/libc.so.6(gsignal+0xc7) [0x7fa64427de97]
        /lib/x86_64-linux-gnu/libc.so.6(abort+0x141) [0x7fa64427f801]
        /lib/x86_64-linux-gnu/libc.so.6(+0x89897) [0x7fa6442c8897]
        /lib/x86_64-linux-gnu/libc.so.6(+0x9090a) [0x7fa6442cf90a]
        /lib/x86_64-linux-gnu/libc.so.6(cfree+0x4cc) [0x7fa6442d6e1c]
        [0x408b3220]

Debug info from gdb:

[New LWP 26391]
[New LWP 26392]
[New LWP 26393]
[New LWP 26394]
[New LWP 26404]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007fa644353bf9 in __GI___poll (fds=0x55cdb5b49940, nfds=3, timeout=500) at ../sysdeps/unix/sysv/linux/poll.c:29
29 ../sysdeps/unix/sysv/linux/poll.c: No such file or directory.
  Id Target Id Frame
* 1 Thread 0x7fa64541a780 (LWP 26388) "Main" 0x00007fa644353bf9 in __GI___poll (fds=0x55cdb5b49940, nfds=3, timeout=500) at ../sysdeps/unix/sysv/linux/poll.c:29
  2 Thread 0x7fa642fff700 (LWP 26391) "SGen worker" 0x00007fa6448559f3 in futex_wait_cancelable (private=<optimized out>, expected=0, futex_word=0x55cdb4788a08) at ../sysdeps/unix/sysv/linux/futex-internal.h:88
  3 Thread 0x7fa6436ee700 (LWP 26392) "Finalizer" 0x00007fa64485a23a in __waitpid (pid=26411, stat_loc=0x7fa6436ec76c, options=0) at ../sysdeps/unix/sysv/linux/waitpid.c:30
  4 Thread 0x7fa633754700 (LWP 26393) "gmain" 0x00007fa644353bf9 in __GI___poll (fds=0x55cdb5550e30, nfds=2, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
  5 Thread 0x7fa632f53700 (LWP 26394) "gdbus" 0x00007fa644353bf9 in __GI___poll (fds=0x55cdb5560ca0, nfds=3, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
  6 Thread 0x7fa628a57700 (LWP 26404) "pool" syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38

Thread 6 (Thread 0x7fa628a57700 (LWP 26404)):
#0 syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
#1 0x00007fa639c2e29a in g_cond_wait_until () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2 0x00007fa639bbb4f1 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3 0x00007fa639bbbaac in g_async_queue_timeout_pop () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#4 0x00007fa639c10bae in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#5 0x00007fa639c10105 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#6 0x00007fa64484f6db in start_thread (arg=0x7fa628a57700) at pthread_create.c:463
#7 0x00007fa64436088f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 5 (Thread 0x7fa632f53700 (LWP 26394)):
#0 0x00007fa644353bf9 in __GI___poll (fds=0x55cdb5560ca0, nfds=3, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1 0x00007fa639be8539 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2 0x00007fa639be88d2 in g_main_loop_run () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3 0x00007fa63a638026 in ?? () from /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0
#4 0x00007fa639c10105 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#5 0x00007fa64484f6db in start_thread (arg=0x7fa632f53700) at pthread_create.c:463
#6 0x00007fa64436088f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 4 (Thread 0x7fa633754700 (LWP 26393)):
#0 0x00007fa644353bf9 in __GI___poll (fds=0x55cdb5550e30, nfds=2, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1 0x00007fa639be8539 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2 0x00007fa639be864c in g_main_context_iteration () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3 0x00007fa639be8691 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#4 0x00007fa639c10105 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#5 0x00007fa64484f6db in start_thread (arg=0x7fa633754700) at pthread_create.c:463
#6 0x00007fa64436088f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 3 (Thread 0x7fa6436ee700 (LWP 26392)):
#0 0x00007fa64485a23a in __waitpid (pid=26411, stat_loc=0x7fa6436ec76c, options=0) at ../sysdeps/unix/sysv/linux/waitpid.c:30
#1 0x000055cdb42955f0 in ?? ()
#2 <signal handler called>
#3 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#4 0x00007fa64427f801 in __GI_abort () at abort.c:79
#5 0x00007fa6442c8897 in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7fa6443f5b9a "%s\n") at ../sysdeps/posix/libc_fatal.c:181
#6 0x00007fa6442cf90a in malloc_printerr (str=str@entry=0x7fa6443f3d88 "free(): invalid pointer") at malloc.c:5350
#7 0x00007fa6442d6e1c in _int_free (have_lock=0, p=0x55cdb5a18e80, av=0x7fa64462ac40 <main_arena>) at malloc.c:4157
#8 __GI___libc_free (mem=0x55cdb5a18e90) at malloc.c:3124
#9 0x00000000408b3220 in ?? ()
#10 0x000055cdb5a18e90 in ?? ()
#11 0x000055cdb51e8190 in ?? ()
#12 0x000055cdb51e8190 in ?? ()
#13 0x0000000000000000 in ?? ()

Thread 2 (Thread 0x7fa642fff700 (LWP 26391)):
#0 0x00007fa6448559f3 in futex_wait_cancelable (private=<optimized out>, expected=0, futex_word=0x55cdb4788a08) at ../sysdeps/unix/sysv/linux/futex-internal.h:88
#1 __pthread_cond_wait_common (abstime=0x0, mutex=0x55cdb4788a20, cond=0x55cdb47889e0) at pthread_cond_wait.c:502
#2 __pthread_cond_wait (cond=0x55cdb47889e0, mutex=0x55cdb4788a20) at pthread_cond_wait.c:655
#3 0x000055cdb441d863 in ?? ()
#4 0x00007fa64484f6db in start_thread (arg=0x7fa642fff700) at pthread_create.c:463
#5 0x00007fa64436088f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7fa64541a780 (LWP 26388)):
#0 0x00007fa644353bf9 in __GI___poll (fds=0x55cdb5b49940, nfds=3, timeout=500) at ../sysdeps/unix/sysv/linux/poll.c:29
#1 0x00007fa639be8539 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2 0x00007fa639be88d2 in g_main_loop_run () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3 0x00007fa63baeda37 in gtk_main () from /usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0
#4 0x000000004089ad2a in ?? ()
#5 0x00007fffbc011dd0 in ?? ()
#6 0x00007fffbc011dc0 in ?? ()
#7 0x00007fa643000950 in ?? ()
#8 0x00007fa643000d60 in ?? ()
#9 0x00007fa643000db0 in ?? ()
#10 0x000055cdb521ae20 in ?? ()
#11 0x000000004077e6d0 in ?? ()
#12 0x00007fffbc011950 in ?? ()
#13 0x00007fffbc011800 in ?? ()
#14 0x000000004089acbc in ?? ()
#15 0x00007fffbc011950 in ?? ()
#16 0x000000004077e688 in ?? ()
#17 0x00007fa6430021f0 in ?? ()
#18 0x00007fa643000788 in ?? ()
#19 0x00007fa6430021f0 in ?? ()
#20 0x00007fa645314130 in ?? ()
#21 0x00007fa643000788 in ?? ()
#22 0x00007fa6430021f0 in ?? ()
#23 0x00007fa645310130 in ?? ()
#24 0x00007fa643000788 in ?? ()
#25 0x00007fa6430021f0 in ?? ()
#26 0x00007fa645308130 in ?? ()
#27 0x00007fa643000400 in ?? ()
#28 0x0000000000000000 in ?? ()

=================================================================
Got a SIGABRT while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries
used by your application.
=================================================================

[1] 26388 abort (core dumped) pinta
1
```

Version information:
- The Pinta version is 1.6.
- I use Ubuntu 18.04.
- `uname -a` gives "Linux stian-Precision-5520 4.15.0-46-generic #49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux".

Is there anything more I can provide to help solve this?

Thanks!