~/.nanorc contents:

syntax "txt" "\.txt"
color green "hi"
formatter ""

Steps to reproduce:
1 - nano foo1.txt
2 - Type "a"
3 - Ctrl + t then Ctrl + o to trigger the crash

Backtrace:

Program received signal SIGSEGV, Segmentation fault.
0x000063895a82ea00 in construct_argument_list (arguments=0x63895a84b7a0 <arguments>, command=0x63895b27efc0 "", filename=0x63895b283d60 "/tmp/nano.T9r5vD.txt") at text.c:2041
2041                (*arguments)[count - 2] = filename;
(gdb) p *arguments
$1 = (char **) 0x0
(gdb) bt
#0  0x000063895a82ea00 in construct_argument_list (arguments=0x63895a84b7a0 <arguments>, command=0x63895b27efc0 "", filename=0x63895b283d60 "/tmp/nano.T9r5vD.txt") at text.c:2041
#1  0x000063895a82ed0c in treat (tempfile_name=0x63895b283d60 "/tmp/nano.T9r5vD.txt", theprogram=0x63895b27efc0 "", spelling=false) at text.c:2133
#2  0x000063895a830df3 in do_formatter () at text.c:2947
#3  0x000063895a82201d in acquire_an_answer (actual=0x7ffd1810b634, listed=0x7ffd1810b633, history_list=0x63895a84b3a8 <execute_history>, refresh_func=0x63895a839f59 <edit_refresh>)
    at prompt.c:540
#4  0x000063895a822226 in do_prompt (menu=128, provided=0x63895b283ce0 "", history_list=0x63895a84b3a8 <execute_history>, refresh_func=0x63895a839f59 <edit_refresh>,
    msg=0x63895a83c701 "Command to execute") at prompt.c:596
#5  0x000063895a80faed in insert_a_file_or (execute=true) at files.c:1231
#6  0x000063895a80fecf in do_execute () at files.c:1384
#7  0x000063895a81eda9 in process_a_keystroke () at nano.c:1684
#8  0x000063895a820c0f in main (argc=2, argv=0x7ffd1810c118) at nano.c:2646

Version: 1a3ef7e497ccd5483dd537b17de07b150ebfa252