Merge pull request #167 from Unity-Technologies/siyaoH/1.17/fixAndroid

fix

com.unity.uiwidgets/Runtime/engine/UIWidgetsMessageManager.cs

         readonly Dictionary<string, MethodChannelMessageDelegate> _methodChannelMessageDelegates = 
             new Dictionary<string, MethodChannelMessageDelegate>();
         public static UIWidgetsMessageManager instance {
-            get { return _instance; } 
+            get {
+                if (_instance == null) {
+                    ensureUIWidgetsMessageManagerIfNeeded();
+                }
+                return _instance;
+            } 
         }
 
         internal static void ensureUIWidgetsMessageManagerIfNeeded() {
                 return;
             }
             var managerObj = new GameObject("__UIWidgetsMessageManager");
-            managerObj.AddComponent<UIWidgetsMessageManager>();
+            var component = managerObj.AddComponent<UIWidgetsMessageManager>();
+            _instance = component;
         }
         
 #if UNITY_IOS || UNITY_ANDROID || UNITY_WEBGL

com.unity.uiwidgets/Runtime/engine/UIWidgetsPanel.cs

         }
         
         protected virtual void Update() {
-            UIWidgetsMessageManager.ensureUIWidgetsMessageManagerIfNeeded();
-
             if (!_viewMetricsCallbackRegistered) {
                 _viewMetricsCallbackRegistered = true;
                 UIWidgetsMessageManager.instance?.AddChannelMessageDelegate("ViewportMetricsChanged",

com.unity.uiwidgets/Runtime/material/selectable_text.cs

         }
     }
 
-    class SelectableText : StatefulWidget {
+    public class SelectableText : StatefulWidget {
 
         public SelectableText(
             string data,

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

                 fn();
             }
 
-            _element.markNeedsBuild();
+            _element?.markNeedsBuild();
         }
 
         public virtual void deactivate() {