bevyengine#4241: fix PR feedbacks

crates/bevy_app/src/app.rs

@@ -58,7 +58,7 @@ pub struct App {
     sub_apps: HashMap<Box<dyn AppLabel>, SubApp>,
     /// A private marker to prevent incorrect calls to `App::run()` from `Plugin::build()`
     /// <https://github.com/bevyengine/bevy/issues/4231>
-    is_from_plugin_build: bool,
+    is_building_plugin: bool,
 }
 
 /// Each [`SubApp`] has its own [`Schedule`] and [`World`], enabling a separation of concerns.
@@ -102,7 +102,7 @@ impl App {
             schedule: Default::default(),
             runner: Box::new(run_once),
             sub_apps: HashMap::default(),
-            is_from_plugin_build: false,
+            is_building_plugin: false,
         }
     }
 
@@ -133,8 +133,8 @@ impl App {
         let _bevy_app_run_guard = bevy_app_run_span.enter();
 
         let mut app = std::mem::replace(self, App::empty());
-        if app.is_from_plugin_build {
-            panic!("App::Run() is called from a Plugin::Build(), which might result in other initialization code not run.");
+        if app.is_building_plugin {
+            panic!("App::run() was called from within Plugin::Build(), which is not allowed.");
         }
         let runner = std::mem::replace(&mut app.runner, Box::new(run_once));
         (runner)(app);
@@ -769,9 +769,9 @@ impl App {
         T: Plugin,
     {
         debug!("added plugin: {}", plugin.name());
-        self.is_from_plugin_build = true;
+        self.is_building_plugin = true;
         plugin.build(self);
-        self.is_from_plugin_build = false;
+        self.is_building_plugin = false;
         self
     }
 
@@ -797,8 +797,10 @@ impl App {
     /// ```
     pub fn add_plugins<T: PluginGroup>(&mut self, mut group: T) -> &mut Self {
         let mut plugin_group_builder = PluginGroupBuilder::default();
+        self.is_building_plugin = true;
         group.build(&mut plugin_group_builder);
         plugin_group_builder.finish(self);
+        self.is_building_plugin = false;
         self
     }