In case you ever need it. If you’re searching for something like “deleted login shell Mac can’t open terminal”, this is the post for you.
I just deleted my login shell (because it was installed with homebrew, and I removed homebrew without remembering that I would lose my shell). That stopped me from opening a Terminal window, because it would immediately bomb out as it was unable to open the shell.
Unable to open a normal Terminal window, anyway. In the Shell menu, the “New Command…” item let me run /bin/bash -l, from which I got to a login-like bash shell. Then I could run this command:
chsh -s /bin/zsh
Enter my password, and then I have a normal shell again.
(So I could then install MacPorts, and then change my shell to /opt/local/bin/bash)