My App crashes in app store review but works fine with me

Question

According to appstore review my app crashes on a specific point. How ever i have tried it on a real device as well as all simulators and works completely fine with me. I was sent a crash log and have symbolicated it. Following is the out come :

Exception Type:  EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000001, 0x0000000104dc8750 ChatInnerVC.checkIfCurrentUserBlocked() (in Blatini) (ChatInnerVC.swift:523)
Termination Reason: Namespace SIGNAL, Code 0x5
Terminating Process: exc handler [6106]
Triggered by Thread:  0

Thread 0 name:  Dispatch queue: com.apple.main-thread
Thread 0 Crashed:
0   Blatini                         0x0000000104dc8750 ChatInnerVC.checkIfCurrentUserBlocked() (in Blatini) (ChatInnerVC.swift:523)
1   Blatini                         0x0000000104dc3aec ChatInnerVC.viewDidLoad() (in Blatini) (ChatInnerVC.swift:63)
2   Blatini                         0x0000000104dc3c48 @objc ChatInnerVC.viewDidLoad() (in Blatini) (<compiler-generated>:0)
3   UIKitCore                       0x00000001a44eb748 0x1a40ef000 + 4179784
4   UIKitCore                       0x00000001a44f02fc 0x1a40ef000 + 4199164
5   UIKitCore                       0x00000001a44f06e8 0x1a40ef000 + 4200168
6   UIKitCore                       0x00000001a444c3a8 0x1a40ef000 + 3527592
7   UIKitCore                       0x00000001a446087c 0x1a40ef000 + 3610748
8   UIKitCore                       0x00000001a4461c1c 0x1a40ef000 + 3615772
9   UIKitCore                       0x00000001a4444900 0x1a40ef000 + 3496192
10  UIKitCore                       0x00000001a502f2bc 0x1a40ef000 + 15991484
11  QuartzCore                      0x00000001a7680978 0x1a7529000 + 1407352
12  QuartzCore                      0x00000001a7680db8 0x1a7529000 + 1408440
13  QuartzCore                      0x00000001a769321c 0x1a7529000 + 1483292
14  QuartzCore                      0x00000001a75d7e10 0x1a7529000 + 716304
15  QuartzCore                      0x00000001a76028c4 0x1a7529000 + 891076
16  QuartzCore                      0x00000001a76034b4 0x1a7529000 + 894132
17  CoreFoundation                  0x00000001a09b011c 0x1a0904000 + 704796
18  CoreFoundation                  0x00000001a09aae4c 0x1a0904000 + 683596
19  CoreFoundation                  0x00000001a09ab2dc 0x1a0904000 + 684764
20  CoreFoundation                  0x00000001a09aabc8 0x1a0904000 + 682952
21  GraphicsServices                0x00000001aad8c5cc 0x1aad89000 + 13772
22  UIKitCore                       0x00000001a4b5d744 0x1a40ef000 + 10938180
23  Blatini                         0x0000000104d24950 main (in Blatini) (AppDelegate.swift:23)
24  libdyld.dylib                   0x00000001a0827384 0x1a0826000 + 4996

Following is the function on line 523 :

   // MARK: Check if current user is blocked i.e your self
    func checkIfCurrentUserBlocked(){
        if clientID.isEmpty {
            self.clientID = self.chatModObj.from_user_id!
        }
        //line 523
        apiManager.checkIfCurrentUserIsBlocked(self.clientID as! String, user_to: self.resid as! String, success: { (result) in
            if result == true {
                self.whenIAmBlocked()
            }else{
                print("You are not blocked")
            }
        }) { (error) in
            print("You are not Blocked")
        }

Following is the function in call on line 63 under viewdidload :

 //line 63
        if self.clientName!.isEmpty{
            print("---->", self.chatModObj.from_username)
            self.clientName = self.chatModObj.from_username
        }

I am unable to track the cause of crash since it works really fine with me and from where the app store is complaining crash line 63 self.clientName is never empty.