Avoid panic by only restoring workspace if UI has launched (#18386)

This should fix the `unregistered setting type
workspace::workspace_settings::WorkspaceSettings` panic that came from
inside `restorable_workspace_locations`.

We tracked it down to a possible scenario (we can't recreate it though)
in which `app.on_reopen` is called before the app has finished
launching.

In any case, this check makes sense, because we only want to restore a
workspace in case the whole app has launched with a UI.

Release Notes:

- N/A

Co-authored-by: Bennet <bennet@zed.dev>

crates/zed/src/main.rs

@@ -425,6 +425,12 @@ fn main() {
     app.on_reopen(move |cx| {
         if let Some(app_state) = AppState::try_global(cx).and_then(|app_state| app_state.upgrade())
         {
+            let ui_has_launched = cx
+                .try_global::<AppMode>()
+                .map(|mode| matches!(mode, AppMode::Ui))
+                .unwrap_or(false);
+
+            if ui_has_launched {
                 cx.spawn({
                     let app_state = app_state.clone();
                     |mut cx| async move {
@@ -435,6 +441,7 @@ fn main() {
                 })
                 .detach();
             }
+        }
     });
 
     app.run(move |cx| {