Unattended App crashes

### Platform

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

### In app purchase type

- [ ] Consumable
- [ ] Non-consumable
- [x] Auto-Renewable Subscription - but most likely not relevant
- [ ] Non-Renewing Subscription

### Environment

- [x] Sandbox
- [ ] Production

### Version
067d464

## Report

### Issue summary
App crashes after lengthy time, most likely in combination with macOS "sleeping"

### What did you expect to happen
no crash. :-)

### What happened instead
I am developing an App that is using SwiftyStoreKit (which is great! - thanks for the good work).
I regularly leave the App under development running (started from Xcode) when I close my MacBook. When I come back it has often crashed. See the 2 examples below.

Happy to provide more information if needed.

#### 1st example
*** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]
(null)

(lldb) bt
* thread #41, queue = 'com.apple.NSXPCConnection.m-user.com.apple.storeagent.storekit', stop reason = breakpoint 1.1
* frame #0: 0x00007fff5f1f16f0 libobjc.A.dylib`objc_exception_throw
frame #1: 0x00007fff3331fa38 CoreFoundation`_CFThrowFormattedException + 202
frame #2: 0x00007fff331ef39e CoreFoundation`-[__NSPlaceholderArray initWithObjects:count:] + 230
frame #3: 0x00007fff3325cc4c CoreFoundation`+[NSArray arrayWithObjects:count:] + 52
frame #4: 0x00007fff3f2b3719 StoreKit`__32-[SKPaymentQueue downloadAdded:]_block_invoke + 120
frame #5: 0x00007fff332759e4 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 219
frame #6: 0x00007fff3f2b3656 StoreKit`-[SKPaymentQueue downloadAdded:] + 286
frame #7: 0x00007fff3f2b63aa StoreKit`-[SKServiceClient downloadAdded:] + 318
frame #8: 0x00007fff3559b79b Foundation`__NSXPCCONNECTION_IS_CALLING_OUT_TO_EXPORTED_OBJECT_S1__ + 12
frame #9: 0x00007fff35594c7e Foundation`-[NSXPCConnection _decodeAndInvokeMessageWithEvent:flags:] + 2631
frame #10: 0x00007fff35593fcc Foundation`message_handler + 227
frame #11: 0x00007fff604f1a50 libxpc.dylib`_xpc_connection_call_event_handler + 56
frame #12: 0x00007fff604ef9e6 libxpc.dylib`_xpc_connection_mach_event + 933
frame #13: 0x000000010167c893 libdispatch.dylib`_dispatch_client_callout4 + 9
frame #14: 0x00000001016980c1 libdispatch.dylib`_dispatch_mach_msg_invoke + 471
frame #15: 0x00000001016847f9 libdispatch.dylib`_dispatch_lane_serial_drain + 279
frame #16: 0x00000001016993ea libdispatch.dylib`_dispatch_mach_invoke + 557
frame #17: 0x00000001016847f9 libdispatch.dylib`_dispatch_lane_serial_drain + 279
frame #18: 0x00000001016858e1 libdispatch.dylib`_dispatch_lane_invoke + 497
frame #19: 0x0000000101690f6c libdispatch.dylib`_dispatch_workloop_worker_thread + 691
frame #20: 0x00000001016f7058 libsystem_pthread.dylib`_pthread_wqthread + 409
frame #21: 0x00000001016f6e51 libsystem_pthread.dylib`start_wqthread + 13

#### 2nd example
*** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]
(null)

objc[8824]: Attempt to use unknown class 0x600000dd3630.
105553170013504

(lldb) bt
* thread #156, queue = 'com.apple.NSXPCConnection.m-user.com.apple.storeagent.storekit', stop reason = breakpoint 1.2
* frame #0: 0x00007fff649f2b48 libc++abi.dylib`__cxa_throw
frame #1: 0x00007fff661fb85a libobjc.A.dylib`objc_exception_throw + 362
frame #2: 0x00007fff3a1df458 CoreFoundation`_CFThrowFormattedException + 202
frame #3: 0x00007fff3a0b10ee CoreFoundation`-[__NSPlaceholderArray initWithObjects:count:] + 230
frame #4: 0x00007fff3a11e99c CoreFoundation`+[NSArray arrayWithObjects:count:] + 52
frame #5: 0x00007fff4619e719 StoreKit`__32-[SKPaymentQueue downloadAdded:]_block_invoke + 120
frame #6: 0x00007fff3a137734 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 219
frame #7: 0x00007fff4619e656 StoreKit`-[SKPaymentQueue downloadAdded:] + 286
frame #8: 0x00007fff461a13aa StoreKit`-[SKServiceClient downloadAdded:] + 318
frame #9: 0x00007fff3c46357b Foundation`__NSXPCCONNECTION_IS_CALLING_OUT_TO_EXPORTED_OBJECT_S1__ + 12
frame #10: 0x00007fff3c45ca5e Foundation`-[NSXPCConnection _decodeAndInvokeMessageWithEvent:flags:] + 2631
frame #11: 0x00007fff3c45bdac Foundation`message_handler + 227
frame #12: 0x00007fff674fca50 libxpc.dylib`_xpc_connection_call_event_handler + 56
frame #13: 0x00007fff674fa9e6 libxpc.dylib`_xpc_connection_mach_event + 933
frame #14: 0x000000010167f863 libdispatch.dylib`_dispatch_client_callout4 + 9
frame #15: 0x000000010169b0a9 libdispatch.dylib`_dispatch_mach_msg_invoke + 487
frame #16: 0x00000001016877c9 libdispatch.dylib`_dispatch_lane_serial_drain + 279
frame #17: 0x000000010169c3ed libdispatch.dylib`_dispatch_mach_invoke + 557
frame #18: 0x00000001016877c9 libdispatch.dylib`_dispatch_lane_serial_drain + 279
frame #19: 0x00000001016888b1 libdispatch.dylib`_dispatch_lane_invoke + 497
frame #20: 0x0000000101693f44 libdispatch.dylib`_dispatch_workloop_worker_thread + 691
frame #21: 0x00000001016fa078 libsystem_pthread.dylib`_pthread_wqthread + 409
frame #22: 0x00000001016f9e71 libsystem_pthread.dylib`start_wqthread + 13