mute ts warning in settings.ts (#18812)

frontend/src/metabase/lib/settings.ts

@@ -126,6 +126,7 @@ class Settings {
 
   on(key: SettingName, callback: SettingListener) {
     this._listeners[key] = this._listeners[key] || [];
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
     this._listeners[key]!.push(callback);
   }