Update sliver.cs

fix sliver issue

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

     }
 
     public class SliverChildListDelegate : SliverChildDelegate {
+        static readonly Key _defaultNullKey = Key.key("SliverChildListDelegate##DefaultNullKey");
+        
         public SliverChildListDelegate(
             List<Widget> children,
             bool addAutomaticKeepAlives = true,
             this.children = children;
             this.addAutomaticKeepAlives = addAutomaticKeepAlives;
             this.addRepaintBoundaries = addRepaintBoundaries;
-            _keyToIndex = new Dictionary<Key, int>(){{Key.key("null"), 0}};
+            _keyToIndex = new Dictionary<Key, int>{{_defaultNullKey, 0}};
         }
 
         public readonly bool addAutomaticKeepAlives;
                 return null;
             }
             if (!_keyToIndex.ContainsKey(key)) {
-                int index = _keyToIndex.getOrDefault(Key.key("null"));
+                int index = _keyToIndex.getOrDefault(_defaultNullKey);
-                    if (child.key != Key.key("null")) {
+                    if (child.key != _defaultNullKey) {
-                        _keyToIndex[Key.key("null")] = index + 1;
+                        _keyToIndex[_defaultNullKey] = index + 1;
-                _keyToIndex[Key.key("null")] = index;
+                _keyToIndex[_defaultNullKey] = index;
             } else {
                 return _keyToIndex[key];
             }
             if (index < 0 || index >= children.Count)
                 return null;
             Widget child = children[index];
-            Key key = child.key != null? new _SaltedValueKey(child.key) : null; 
+            
-                "The sliver's children must not contain null values, but a null value was found at index $index"
+                    "The sliver's children must not contain null values, but a null value was found at index $index"
+            
+            Key key = child.key != null? new _SaltedValueKey(child.key) : null;
             if (addRepaintBoundaries)
                 child = new RepaintBoundary(child: child);
             if (addAutomaticKeepAlives)