thread 'main' panicked at 'assertion failed: self.is_char_boundary(new_len)', src/liballoc/string.rs:1123:13

Fractal crashed twice for me today (unusual). It dumped core, so that's good: I can report a bug. But sadly the backtrace seems useless:

(gdb) bt full
#0  0x00007ff90277a605 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
        set = 
            {__val = {0, 140701903744376, 94389790872800, 0, 94389789877672, 0, 94389789877672, 94389788413851, 3, 0, 140701903744259, 140701903744104, 0, 94389788601259, 0, 15}}
        pid = <optimized out>
        tid = <optimized out>
#1  0x00007ff902763899 in __GI_abort () at abort.c:79
        save_stage = 1
        act = 
          {__sigaction_handler = {sa_handler = 0x55d8d5e565a8, sa_sigaction = 0x55d8d5e565a8}, sa_mask = {__val = {0, 3, 11796080518979168730, 94389790872579, 4, 0, 94389788452763, 140701903744480, 3, 140701903745025, 94389790872824, 140701903745064, 140701903744904, 0, 94389788493539, 2}}, sa_flags = -1224871896, sa_restorer = 0x55d8d5f494f8}
        sigs = {__val = {32, 0 <repeats 15 times>}}
#2  0x000055d8d5d08dd7 in panic_abort::__rust_start_panic::abort ()
#3  0x0000000000000000 in  ()

I had installed org.gnome.Fractal.Debug and org.gnome.Sdk.Debug before taking this backtrace. So what's required to get a proper rust backtrace from the core?