iOS crash in APP enterbackground and close MQTT

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Observed Behavior

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Stack Trace Please provide a stack trace if applicable e.g. a crash is occurring.

Code Snippet Please provide a snippet of the code causing the issue or how you are using the service that has the bug.

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**Areas of the SDK you are using (AWSCore, AWSIOT)

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Environment(please complete the following information):

SDK Version: [e.g. 2.42.2]
Dependency Manager: [e.g. Cocoapods, Carthage]
Swift Version : [e.g. 4.0]
Xcode Version: [e.g. 11.4]

Device Information (please complete the following information):

Device: iPhone 11
iOS Version: [e.g. iOS 14.2]
Specific to simulators:

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Relevant Console Output In some cases, it might be helpful to add any logs from your console that better help you tell the story of this issue. You can easily enable logging in your app by putting the following code in your app delegate.


<img width="1220" alt="截屏2021-06-05 下午4 56 30" src="https://user-images.githubusercontent.com/7712917/120886143-04198800-c61f-11eb-94b2-e31f04627548.png">

#11 Thread
AWSIoT.  -[AWSIoTMQTTClient cleanUpToDecoderStream] + 108

AWSIoT   -[AWSIoTMQTTClient webSocket:didCloseWithCode:reason:wasClean:] + 204

AWSIoT  __41-[AWSSRWebSocket safeHandleEvent:stream:]_block_invoke_2 + 148

libdispatch.dylib  __dispatch_call_block_and_release + 32

libsystem_pthread.dylib
__pthread_wqthread + 216

aws-amplify / aws-sdk-ios

iOS crash in APP enterbackground and close MQTT #3625