only store runtime platform when requested

src/Markup/Avalonia.Markup.Xaml/XamlIl/Runtime/XamlIlRuntimeHelpers.cs

@@ -50,7 +50,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.Runtime
             private readonly IServiceProvider? _parentProvider;
             private readonly List<IResourceNode>? _parentResourceNodes;
             private readonly INameScope _nameScope;
-            private readonly IRuntimePlatform? _runtimePlatform;
+            private IRuntimePlatform? _runtimePlatform;
 
             public DeferredParentServiceProvider(IServiceProvider? parentProvider, List<IResourceNode>? parentResourceNodes,
                 object rootObject, INameScope nameScope)
@@ -59,7 +59,6 @@ namespace Avalonia.Markup.Xaml.XamlIl.Runtime
                 _parentResourceNodes = parentResourceNodes;
                 _nameScope = nameScope;
                 RootObject = rootObject;
-                _runtimePlatform = AvaloniaLocator.Current.GetService<IRuntimePlatform>();
             }
 
             public IEnumerable<object> Parents => GetParents();
@@ -83,7 +82,11 @@ namespace Avalonia.Markup.Xaml.XamlIl.Runtime
                 if (serviceType == typeof(IAvaloniaXamlIlControlTemplateProvider))
                     return this;
                 if (serviceType == typeof(IRuntimePlatform))
+                {
+                    if(_runtimePlatform == null)
+                        _runtimePlatform = AvaloniaLocator.Current.GetService<IRuntimePlatform>();
                     return _runtimePlatform;
+                }
                 return _parentProvider?.GetService(serviceType);
             }