Update "unregistered_view_type" error message

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

@@ -73,8 +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',
-        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;
     }

lib/web_ui/test/engine/platform_views/message_handler_test.dart

@@ -46,7 +46,8 @@ void testMain() {
             codec.decodeEnvelope(response!);
           } on PlatformException catch (e) {
             expect(e.code, 'unregistered_view_type');
-            expect(e.details, contains(viewType));
+            expect(e.message, contains(viewType));
+            expect(e.details, contains('registerViewFactory'));
           }
         });
 

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

@@ -129,7 +129,8 @@
   NSDictionary<NSString*, id>* args = [call arguments];
 
   long viewId = [args[@"id"] longValue];
-  std::string viewType([args[@"viewType"] UTF8String]);
+  NSString* viewTypeString = args[@"viewType"];
+  std::string viewType(viewTypeString.UTF8String);
 
   if (views_.count(viewId) != 0) {
     result([FlutterError errorWithCode:@"recreating_view"
@@ -139,10 +140,18 @@
 
   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"
-                               details:[NSString stringWithFormat:@"unregistered view type: '%@'",
-                                                                  args[@"viewType"]]]);
+    result([FlutterError
+        errorWithCode:@"unregistered_view_type"
+              message:[NSString stringWithFormat:@"A UIKitView widget is trying to create a "
+                                                 @"PlatformView with an unregistered type: < %@ >",
+                                                 viewTypeString]
+              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#on-the-platform-side-1 for more details.\n"
+                      @"If you are not the author of the PlatformView, make sure to call "
+                      @"`GeneratedPluginRegistrant.register`."]);
     return;
   }
 

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

@@ -39,8 +39,16 @@ - (void)onCreateWithViewID:(int64_t)viewId
   if (!factory) {
     result([FlutterError
         errorWithCode:@"unregistered_view_type"
-              message:@"trying to create a view with an unregistered type"
-              details:[NSString stringWithFormat:@"unregistered view type: '%@'", 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#on-the-platform-side-1 for more details.\n"
+                      @"If you are not the author of the PlatformView, make sure to call "
+                      @"`GeneratedPluginRegistrant.register`."]);
     return;
   }