add missing DispatchQueue.main switch for scale & torch on iOS

@@ -216,9 +216,12 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
         backgroundQueue.async {
             self.captureSession.startRunning()
 
-            // Turn on the flashlight if requested,
-            // but after the capture session started.
+            // After the capture session started, turn on the torch (if requested)
+            // and reset the zoom scale back to the default.
+            // Ensure that these adjustments are done on the main DispatchQueue,
+            // as they interact with the hardware camera.
             if (torch) {
+                DispatchQueue.main.async {
                     do {
                         try self.toggleTorch(.on)
                     } catch {
@@ -226,15 +229,21 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
                         // continue with the capture session anyway.
                     }
                 }
+            }
             
+            DispatchQueue.main.async {
                 do {
                     try self.resetScale()
                 } catch {
                     // If the zoom scale could not be reset,
                     // continue with the capture session anyway.
                 }
+            }
 
             if let device = self.device {
+                // When querying the dimensions of the camera,
+                // stay on the background thread,
+                // as this does not change the configuration of the hardware camera.
                 let dimensions = CMVideoFormatDescriptionGetDimensions(
                     device.activeFormat.formatDescription)
                 let hasTorch = device.hasTorch