WebKitLegacy Crash in iOS - ios

I am getting some crash reports as following on Firebase Crashlytics. Unable to reproduce from our side. Is there any way to check this crash? or any reason?
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TestFlight not providing crash log

For a certain crash I am only getting TestFlight feedback and not the crash log. Meaning I cannot open the crash in Xcode to figure out what the issue is. Is there a reason for this?
How can I get the crash log?

CFNetwork Crash : CFURLRequestSetHTTPRequestBody + 12112

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My app encountered this crash in recent months.From the stacktrace as the following, I can‘t find any clue.This crash only happened on iOS 11.2.x and background thread. This bug almost happened only once on each user's device. I don't know if it is a bug from Apple.Can anybody give me some clues about this bug? Thanks a lot!

iOS app reported to be crashing by some end user. No clue on how to exploit crash report

Some end users report a crash on published application.
In spite of all our efforts, no way to reproduce it. And crash reports from Xcode do not help much.
All that we have is: "FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK"
See joined picture.
see here details of the crash as reported in Xcode

Google Analytics Crash and Exception report

I wrote below code for Google analytics crash report.
GAI.sharedInstance().trackUncaughtExceptions = true
I am trying to generate crash report. I kept some code which is causing App crash but i can't see any report in behavior -> crash and exception.
Could you please suggest?

Could not find crash in Symbolicating iPhone App Crash Reports

Sample Iphone App Crash Report
Hi this is Symbolicating iPhone App Crash Reports image. I could not found where is crash so can u help me find crash
Use Symbolicatecrash tool, you can check the link to know how to use the tool to symbolicate your crash report.
As far as your crash is concern, it happened in thread 0 however to know exactly what went wrong in code, you need to symbolicate your crash because crash log is only showing addresses.

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