fix(ui): 修复UiRouterBase弹出栈时目标UI键可空性问题

- 将topUiKey重命名为leavingUiKey以提高代码可读性
- 修改nextUiKey逻辑使其在栈中只有一个元素时返回null而非抛出异常
- 更新CreateEvent方法参数toUiKey为可空字符串类型
- 调整UiTransitionEvent中的ToUiKey属性为可空字符串类型
- 添加注释说明nextUiKey现在是可选的
- 格式化代码缩进和换行以提高可读性

GFramework.Game.Abstractions/ui/UiTransitionEvent.cs

@@ -22,7 +22,7 @@ public sealed class UiTransitionEvent
     /// <summary>
     /// 目标UI的标识符，切换后的UI key
     /// </summary>
-    public string ToUiKey { get; init; } = string.Empty;
+    public string? ToUiKey { get; init; } = string.Empty;
 
     /// <summary>
     /// UI切换类型

GFramework.Game/ui/UiRouterBase.cs

@@ -14,6 +14,16 @@ public abstract class UiRouterBase : AbstractSystem, IUiRouter
 {
     private static readonly ILogger Log = LoggerFactoryResolver.Provider.CreateLogger("UiRouterBase");
 
+    /// <summary>
+    /// 路由守卫列表
+    /// </summary>
+    private readonly List<IUiRouteGuard> _guards = new();
+
+    /// <summary>
+    /// 层级管理（非栈层级），用于Overlay、Modal、Toast等浮层
+    /// </summary>
+    private readonly Dictionary<UiLayer, Dictionary<string, IUiPageBehavior>> _layers = new();
+
     /// <summary>
     /// UI切换处理器管道
     /// </summary>
@@ -24,11 +34,6 @@ public abstract class UiRouterBase : AbstractSystem, IUiRouter
     /// </summary>
     private readonly Stack<IUiPageBehavior> _stack = new();
 
-    /// <summary>
-    /// 层级管理（非栈层级），用于Overlay、Modal、Toast等浮层
-    /// </summary>
-    private readonly Dictionary<UiLayer, Dictionary<string, IUiPageBehavior>> _layers = new();
-
     /// <summary>
     /// UI工厂实例，用于创建UI相关的对象
     /// </summary>
@@ -36,11 +41,6 @@ public abstract class UiRouterBase : AbstractSystem, IUiRouter
 
     private IUiRoot _uiRoot = null!;
 
-    /// <summary>
-    /// 路由守卫列表
-    /// </summary>
-    private readonly List<IUiRouteGuard> _guards = new();
-
     /// <summary>
     /// 注册UI切换处理器
     /// </summary>
@@ -143,19 +143,23 @@ public abstract class UiRouterBase : AbstractSystem, IUiRouter
             return;
         }
 
-        var topUiKey = _stack.Peek().Key;
+        var leavingUiKey = _stack.Peek().Key;
 
-        // 执行离开守卫
+        if (!ExecuteLeaveGuardsAsync(leavingUiKey).GetAwaiter().GetResult())
-        if (!ExecuteLeaveGuardsAsync(topUiKey).GetAwaiter().GetResult())
         {
-            Log.Warn("Pop blocked by guard: {0}", topUiKey);
+            Log.Warn("Pop blocked by guard: {0}", leavingUiKey);
             return;
         }
 
+        // ⚠️ 注意：nextUiKey 现在是可选的
         var nextUiKey = _stack.Count > 1
-            ? _stack.ElementAt(1).Key // 使用 Key 而不是 View.GetType().Name
+            ? _stack.ElementAt(1).Key
-            : throw new InvalidOperationException("Stack is empty");
+            : null;
-        var @event = CreateEvent(nextUiKey, UiTransitionType.Pop);
+
+        var @event = CreateEvent(
+            nextUiKey,
+            type: UiTransitionType.Pop
+        );
 
         BeforeChange(@event);
 
@@ -164,6 +168,7 @@ public abstract class UiRouterBase : AbstractSystem, IUiRouter
         AfterChange(@event);
     }
 
+
     /// <summary>
     /// 替换当前所有页面为新页面（基于uiKey）
     /// </summary>
@@ -312,7 +317,7 @@ public abstract class UiRouterBase : AbstractSystem, IUiRouter
     /// 创建UI切换事件
     /// </summary>
     private UiTransitionEvent CreateEvent(
-        string toUiKey,
+        string? toUiKey,
         UiTransitionType type,
         UiTransitionPolicy? policy = null,
         IUiPageEnterParam? param = null