minor fix on threading issue

Session/Shared/OWSQRCodeScanningViewController.m

@@ -115,7 +115,10 @@ NS_ASSUME_NONNULL_BEGIN
                 self.capture.layer.frame = self.view.bounds;
                 [self.view.layer addSublayer:self.capture.layer];
                 [self.view bringSubviewToFront:self.maskingView];
-                [self.capture start];
+                
+                dispatch_async(self.captureQueue, ^{
+                    [self.capture start];
+                });
             });
         });
     } else {

Session/Shared/ScanQRCodeWrapperVC.swift

@@ -67,9 +67,7 @@ final class ScanQRCodeWrapperVC : BaseVC {
     override func viewDidAppear(_ animated: Bool) {
         super.viewDidAppear(animated)
         UIDevice.current.ows_setOrientation(.portrait)
-        DispatchQueue.main.async { [weak self] in
+        self.scanQRCodeVC.startCapture()
-            self?.scanQRCodeVC.startCapture()
-        }
     }
     
     // MARK: Interaction
@@ -78,8 +76,6 @@ final class ScanQRCodeWrapperVC : BaseVC {
     }
     
     public func startCapture() {
-        DispatchQueue.main.async { [weak self] in
+        self.scanQRCodeVC.startCapture()
-            self?.scanQRCodeVC.startCapture()
-        }
     }
 }