升级topon sdk版本2.1.5， 升级平台插件版本至最新

AnyThinkPlugin/AnyThinkAds/Platform/iOS/Internal/C/ATUnityUtilities.m

@@ -23,6 +23,16 @@ NSString *const kATUnityCheckLoadModelAdInfoKey = @"adInfo";
 @end
 @implementation NSDictionary (KAKit)
 -(NSString*) jsonString {
+
+    @try {
+        // can not serialization, return a value is null array string
+        if (![NSJSONSerialization isValidJSONObject:self]) {
+            return @"[]";
+        }
+    } @catch (NSException *exception) {
+        return @"[]";
+    } @finally {}
+    
     NSError *error;
     NSData *jsonData;
     @try {