Plugin API: open_terminal fails when default_shell is not set

[SecretPocketCat]

2. Issues with the Zellij UI / behavior / crash

zellij --version:
zellij 0.41.0 (built from main)

uname -av or ver(Windows):
Linux archrig 6.8.1-arch1-1 #1 SMP PREEMPT_DYNAMIC Sat, 16 Mar 2024 17:15:35 +0000 x86_64 GNU/Linux

Issue description

Opening a terminal pane from a plugin using the open_terminal command fails when default_shell is unset. (The same happens with open_terminal_floating, but open_command_pane works fine.

This code

open_terminal(get_plugin_ids().initial_cwd);

leads to this ERROR log

ERROR  |zellij_server::pty       | 2024-04-16 22:52:23.462 [pty       ] [zellij-server/src/pty.rs:213]: Failed to spawn terminal: failed to spawn terminal for ClientId(1)

Caused by:
    0: failed to spawn terminal for ClientId(1)
    1: failed to spawn terminal
    2: failed to open PTY for command ''
    3: could not find command '' for terminal 5

Minimal reproduction

1. Plugin code:

use std::collections::BTreeMap;
use zellij_tile::prelude::*;

#[derive(Default)]
struct PluginState {}

register_plugin!(PluginState);

impl ZellijPlugin for PluginState {
    fn load(&mut self, _configuration: BTreeMap<String, String>) {
        request_permission(&[PermissionType::OpenTerminalsOrPlugins]);
        subscribe(&[EventType::Key]);
    }

    fn update(&mut self, event: Event) -> bool {
        match event {
            Event::Key(Key::Char('t')) => {
                open_terminal(get_plugin_ids().initial_cwd);
            }
            _ => unimplemented!("{event:?}"),
        };

        false
    }
}

2. Layout:

layout {
    pane {
        plugin location="file:target/wasm32-wasi/release/terminal_pane_repro.wasm"
    }
}  

3. Start zellij:
   zellij --layout workspace.kdl

4. Press t, but nothing happens

Other relevant information

Setting default_shell (to fish in my case) fixed the issue and terminal panes are spawning fine with that setting.

Running echo $SHELL in a terminal pane spawned by the layout:

echo $SHELL 
/usr/bin/fish