Build null error

I might be doing something wrong but when I'm trying to build this app using your instructions, after ./make.sh I get
```
./newns.c: In function ‘main’:
./newns.c:19:10: warning: null argument where non-null required (argument 2) [-Wnonnull]
   19 |   return execv("/bin/sh", NULL);
      |          ^~~~~
```

Any clues how to fix this please?