Very unstable 1.6 on Lubuntu-18.04

Pinta-1.6 on Lubuntu-18.04
Freezes on trying to mark a selection box on 20 megapixel photo (4.9 MB), sometimes before selecting the tool.
Why does it need libpthread?

dk@i7:~$ pinta

(Pinta:26865): GLib-GIO-CRITICAL **: 11:34:27.644: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed

(Pinta:26865): GLib-GIO-CRITICAL **: 11:34:27.644: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed
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) [0x5605dd8f2514]
 /lib/x86_64-linux-gnu/libpthread.so.0(+0x12890) [0x7efdfa91b890]
 /lib/x86_64-linux-gnu/libc.so.6(gsignal+0xc7) [0x7efdfa33ee97]
 /lib/x86_64-linux-gnu/libc.so.6(abort+0x141) [0x7efdfa340801]
 /lib/x86_64-linux-gnu/libc.so.6(+0x89897) [0x7efdfa389897]
 /lib/x86_64-linux-gnu/libc.so.6(+0x9090a) [0x7efdfa39090a]
 /lib/x86_64-linux-gnu/libc.so.6(cfree+0x4cc) [0x7efdfa397e1c]
 [0x413d3160]

Debug info from gdb:

=================================================================
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.
=================================================================

Aborted (core dumped)
dk@i7:~$