replace c str with NSString

Author: dacianf

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

@@ -129,6 +129,7 @@
   NSDictionary<NSString*, id>* args = [call arguments];
 
   long viewId = [args[@"id"] longValue];
+  NSString* viewTypeString = args[@"viewType"];
   std::string viewType([args[@"viewType"] UTF8String]);
 
   if (views_.count(viewId) != 0) {
@@ -142,8 +143,8 @@
     result([FlutterError
         errorWithCode:@"unregistered_view_type"
               message:[NSString stringWithFormat:@"A UIKitView widget is trying to create a "
-                                                 @"PlatformView with an unregistered type: < %s >",
-                                                 viewType.c_str()]
+                                                 @"PlatformView with an unregistered type: < %@ >",
+                                                 viewTypeString]
               details:@"If you are the author of the PlatformView, make sure `registerViewFactory`"
                       @"is invoked.\n"
                       @"See: "