Update "unregistered_view_type" error message based on PR review

Author: dacianf

lib/web_ui/lib/src/engine/platform_views/message_handler.dart

@@ -73,9 +73,10 @@ class PlatformViewMessageHandler {
     if (!_contentManager.knowsViewType(viewType)) {
       callback(_codec.encodeErrorEnvelope(
         code: 'unregistered_view_type',
-        message: 'trying to create a view with an unregistered type. '
-            'Have you called GeneratedPluginRegistrant.register?',
-        details: 'unregistered view type: $viewType',
+        message: 'A HtmlElementView widget is trying to create a platform view'
+            ' with an unregistered type: <$viewType>.',
+        details: 'If you are the author of the PlatformView, make sure '
+            '`registerViewFactory` is invoked.',
       ));
       return;
     }

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -140,10 +140,11 @@
   NSObject<FlutterPlatformViewFactory>* factory = factories_[viewType].get();
   if (factory == nil) {
     result([FlutterError errorWithCode:@"unregistered_view_type"
-                               message:@"trying to create a view with an unregistered type. Have "
-                                       @"you called GeneratedPluginRegistrant.register?"
-                               details:[NSString stringWithFormat:@"unregistered view type: '%@'",
-                                                                  args[@"viewType"]]]);
+                               message:[NSString stringWithFormat:@"A UIKitView widget is trying to create a PlatformView with an unregistered type: < %@ >", viewType]
+                               details:@"If you are the author of the PlatformView, make sure `registerViewFactory` is invoked.\n"
+                                       @"See: https://docs.flutter.dev/development/platform-integration/platform-views?tab=ios-platform-views-objective-c-tab#on-the-platform-side-1 for more details."
+                                       @"If you are not the author of the PlatformView, make sure to call "
+                                       @"`GeneratedPluginRegistrant.register`."]);
     return;
   }
 

shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.mm

@@ -37,11 +37,12 @@ - (void)onCreateWithViewID:(int64_t)viewId
 
   NSObject<FlutterPlatformViewFactory>* factory = _platformViewFactories[viewType];
   if (!factory) {
-    result([FlutterError
-        errorWithCode:@"unregistered_view_type"
-              message:@"trying to create a view with an unregistered type. Have you called "
-                      @"GeneratedPluginRegistrant.register?"
-              details:[NSString stringWithFormat:@"unregistered view type: '%@'", viewType]]);
+    result([FlutterError errorWithCode:@"unregistered_view_type"
+                                message:[NSString stringWithFormat:@"A UIKitView widget is trying to create a PlatformView with an unregistered type: < %@ >", viewType]
+                                details:@"If you are the author of the PlatformView, make sure `registerViewFactory` is invoked.\n"
+                                        @"See: https://docs.flutter.dev/development/platform-integration/platform-views?tab=ios-platform-views-objective-c-tab#on-the-platform-side-1 for more details."
+                                        @"If you are not the author of the PlatformView, make sure to call "
+                                        @"`GeneratedPluginRegistrant.register`."]);
     return;
   }