fix some obvious typos :)

Runtime/engine/UIWidgetsPanel.cs

             this.material = mat;
         }
 
-        void Update() {
+        protected virtual void Update() {
             PerformanceUtils.instance.updateDeltaTime(Time.unscaledDeltaTime);
             this._displayMetrics.Update();
             UIWidgetsMessageManager.ensureUIWidgetsMessageManagerIfNeeded();

Runtime/material/popup_menu.cs

     }
 
     public static partial class PopupMenuUtils {
-        public static IPromise<T> showMenu<T>(
+        public static IPromise<object> showMenu<T>(
             BuildContext context,
             RelativeRect position,
             List<PopupMenuEntry<T>> items,
                 initialValue: initialValue,
                 elevation: elevation,
                 theme: Theme.of(context, shadowThemeOnly: true)
-            )).Then(result => (T) result);
+            ));
         }
     }
 
                 ),
                 Offset.zero & overlay.size
             );
-            PopupMenuUtils.showMenu<T>(
+            PopupMenuUtils.showMenu(
                     context: this.context,
                     elevation: this.widget.elevation,
                     items: this.widget.itemBuilder(this.context),
                     }
 
                     if (this.widget.onSelected != null) {
-                        this.widget.onSelected(newValue);
+                        this.widget.onSelected((T) newValue);
                     }
                 });
         }

Runtime/rendering/sliver_persistent_header.cs

             Curve curve = null,
             TimeSpan? duration = null
         ) {
-            D.assert(this.vsync != null);
+            D.assert(vsync != null);
             this.vsync = vsync;
             this.curve = curve ?? Curves.ease;
             this.duration = duration ?? new TimeSpan(0, 0, 0, 0, 300);

Runtime/widgets/binding.cs

                 element.unmount();
             }
 
-            this._renderViewElement.visitChildren(unMountAll);
-            this._renderViewElement.deactivate();
-            this._renderViewElement.unmount();
+            if (this._renderViewElement != null) {
+                this._renderViewElement.visitChildren(unMountAll);
+                this._renderViewElement.deactivate();
+                this._renderViewElement.unmount();
+                this._renderViewElement = null;
+            }
         }
 
         public void attachRootWidget(Widget rootWidget) {

Runtime/widgets/page_storage.cs

             }
 
             if (identifier != null) {
-                return this._storage[identifier];
+                return this._storage.getOrDefault(identifier);
-            return contextIdentifier.isNotEmpty ? this._storage[contextIdentifier] : null;
+            return contextIdentifier.isNotEmpty ? this._storage.getOrDefault(contextIdentifier) : null;
         }
     }
 

Runtime/widgets/single_child_scroll_view.cs

                 }
 
                 if (this.attached) {
-                    this.offset.removeListener(this._hasScrolled);
+                    this._offset.removeListener(this._hasScrolled);
-                this.offset = value;
+                this._offset = value;
                 if (this.attached) {
                     this._offset.addListener(this._hasScrolled);
                 }