Remmina crashes using the Mir GTK backend.
Initially I would have blamed the gtk/gdk libraries, but it appears Remmina itself is making an X11-only call directly that's the problem:
(gdb) bt
#0 0x00007f74ddb41bd9 in raise (sig=11)
at ../sysdeps/unix/sysv/linux/pt-raise.c:36
#1 0x00007f74cbdfb555 in ?? ()
from /usr/lib/x86_64-linux-gnu/libfreerdp-utils.so.1.1
#2 <signal handler called>
#3 0x00007f74df971610 in ?? () from /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#4 0x00007f74df154a93 in g_hash_table_lookup ()
from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#5 0x00007f74df47f2f4 in gdk_x11_atom_to_xatom_for_display ()
from /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#6 0x0000000000435cea in remmina_public_get_current_workspace ()
#7 0x000000000043de41 in remmina_widget_pool_find ()
#8 0x000000000041f36f in remmina_exec_command ()
Here's a patch that stops the crashing on non-X11 platforms.