Crash in ProductsInfoController.retrieveProductsInfo

### Platform

- [x] iOS
- [ ] macOS
- [ ] tvOS

### In app purchase type

- [ ] Consumable
- [x] Non-consumable
- [ ] Auto-Renewable Subscription
- [ ] Non-Renewing Subscription

### Environment

- [x] Sandbox
- [x] Production

### Version
0.15.0

## Report

### Issue summary

sometimes, it crash when it retrieve products info.

```0 SwiftyStoreKit 0x1014a6db4 closure #1 in ProductsInfoController.retrieveProductsInfo(_:completion:) ()
1 SwiftyStoreKit 0x101497258 partial apply for closure #1 in InAppProductQueryRequest.performCallback(_:) (InAppProductQueryRequest.swift:76)
2 SwiftyStoreKit 0x1014a0f34 thunk for @escaping @callee_guaranteed () -> () ()
3 libdispatch.dylib 0x2320bca38 _dispatch_call_block_and_release + 24
4 libdispatch.dylib 0x2320bd7d4 _dispatch_client_callout + 16
5 libdispatch.dylib 0x23209d9e4 _dispatch_main_queue_callback_4CF$VARIANT$armv81 + 1008
6 CoreFoundation 0x23260dec0 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12
7 CoreFoundation 0x232608df8 __CFRunLoopRun + 1924
8 CoreFoundation 0x232608354 CFRunLoopRunSpecific + 436
9 GraphicsServices 0x23480879c GSEventRunModal + 104
10 UIKitCore 0x25ebf3b68 UIApplicationMain + 212
11 app. 0x100438400 main (main.m:14)
12 libdyld.dylib 0x2320ce8e0 start + 4```