SHELL: do not index an empty selection when the command starts

`bool::then_some` evaluates its argument eagerly, so starting SHELL or
SOLIDEDIT with nothing selected indexed an empty vector and aborted the
application. The single-solid target now comes from a slice match.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/app/commands/draw.rs

@@ -2006,7 +2006,13 @@ impl OpenCADStudio {
                         )
                     })
                     .collect::<Vec<_>>();
-                let target = (selected.len() == 1).then_some(selected[0]);
+                // `then_some` evaluates its argument eagerly: indexing an empty
+                // selection here crashed the app when SHELL was started with
+                // nothing selected.
+                let target = match selected.as_slice() {
+                    [handle] => Some(*handle),
+                    _ => None,
+                };
                 let new_cmd = if cmd == "SHELL" {
                     ShellCommand::direct(target)
                 } else {