fix sliver persistent header bug

com.unity.uiwidgets/Runtime/widgets/sliver_persistent_header.cs

 using Unity.UIWidgets.rendering;
 
 namespace Unity.UIWidgets.widgets {
+    
+    static class sliver_persistent_header_utils {
+       public static _SliverPersistentHeaderElement _element { get; set; }
+    }
+    
     public abstract class SliverPersistentHeaderDelegate {
         public SliverPersistentHeaderDelegate() {
         }
 
         public override void mount(Element parent, object newSlot) {
             base.mount(parent, newSlot);
-            (renderObject as _RenderSliverPersistentHeaderForWidgetsMixin)._element = this;
+            sliver_persistent_header_utils._element = this;
-            (renderObject as _RenderSliverPersistentHeaderForWidgetsMixin)._element = null;
+            sliver_persistent_header_utils._element = null;
         }
 
         public override void update(Widget _newWidget) {
     }
 
     public interface _RenderSliverPersistentHeaderForWidgetsMixin {
-        _SliverPersistentHeaderElement _element { get; set; }
 
         void triggerRebuild();
     }
         }
 
         public _SliverPersistentHeaderElement _element {
-            get { return _ele; }
-            set { _ele = value; }
+            get { return sliver_persistent_header_utils._element; }
+            set { sliver_persistent_header_utils._element = value; }
         }
 
         _SliverPersistentHeaderElement _ele;

com.unity.uiwidgets/Runtime/widgets/sliver_persistent_header.mixin.gen.cs

             _snapConfiguration = snapConfiguration;
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }
             stretchConfiguration: stretchConfiguration) {
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }
             stretchConfiguration: stretchConfiguration) {
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }
             stretchConfiguration: stretchConfiguration) {
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }

com.unity.uiwidgets/Runtime/widgets/sliver_persistent_header.mixin.njk

             _snapConfiguration = snapConfiguration;
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }
             stretchConfiguration: stretchConfiguration) {
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }
             stretchConfiguration: stretchConfiguration) {
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }
             stretchConfiguration: stretchConfiguration) {
         }
 
-         public _SliverPersistentHeaderElement _element { get; set; }
+         public _SliverPersistentHeaderElement _element {
+             get {
+                 return sliver_persistent_header_utils._element;
+             }
+             set {
+                 if (value == sliver_persistent_header_utils._element) {
+                     return;
+                 }
+ 
+                 sliver_persistent_header_utils._element = value;
+             }
+             
+         }
-         float? minExtent {
+         public override float? minExtent {
-         float? maxExtent {
+         public override float? maxExtent {
              get {
                  return _element.widget.layoutDelegate.minExtent;
              }