fix nil capture session bug

+## 4.0.1
+Bugs fixed:
+* [iOS] Fixed a crash with a nil capture session when starting the camera. (thanks @navaronbracke !)
+
 ## 4.0.0
 BREAKING CHANGES:
 * [Android] compileSdk has been upgraded to version 34.

@@ -17,7 +17,7 @@ typealias ZoomScaleChangeCallback = ((Double?) -> ())
 
 public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelegate, FlutterTexture {
     /// Capture session of the camera
-    var captureSession: AVCaptureSession!
+    var captureSession: AVCaptureSession?
 
     /// The selected camera
     var device: AVCaptureDevice!
@@ -173,7 +173,7 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
     /// Start scanning for barcodes
     func start(barcodeScannerOptions: BarcodeScannerOptions?, returnImage: Bool, cameraPosition: AVCaptureDevice.Position, torch: Bool, detectionSpeed: DetectionSpeed, completion: @escaping (MobileScannerStartParameters) -> ()) throws {
         self.detectionSpeed = detectionSpeed
-        if (device != nil) {
+        if (device != nil || captureSession != nil) {
             throw MobileScannerError.alreadyStarted
         }
 
@@ -216,17 +216,17 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
             device.unlockForConfiguration()
         } catch {}
 
-        captureSession.beginConfiguration()
+        captureSession!.beginConfiguration()
 
         // Add device input
         do {
             let input = try AVCaptureDeviceInput(device: device)
-            captureSession.addInput(input)
+            captureSession!.addInput(input)
         } catch {
             throw MobileScannerError.cameraError(error)
         }
 
-        captureSession.sessionPreset = AVCaptureSession.Preset.photo
+        captureSession!.sessionPreset = AVCaptureSession.Preset.photo
         // Add video output.
         let videoOutput = AVCaptureVideoDataOutput()
 
@@ -237,17 +237,21 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
         // calls captureOutput()
         videoOutput.setSampleBufferDelegate(self, queue: DispatchQueue.main)
 
-        captureSession.addOutput(videoOutput)
+        captureSession!.addOutput(videoOutput)
         for connection in videoOutput.connections {
             connection.videoOrientation = .portrait
             if cameraPosition == .front && connection.isVideoMirroringSupported {
                 connection.isVideoMirrored = true
             }
         }
-        captureSession.commitConfiguration()
+        captureSession!.commitConfiguration()
 
         backgroundQueue.async {
-            self.captureSession.startRunning()
+            guard let captureSession = self.captureSession else {
+                return
+            }
+
+            captureSession.startRunning()
 
             // After the capture session started, turn on the torch (if requested)
             // and reset the zoom scale back to the default.
@@ -299,15 +303,16 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
 
     /// Stop scanning for barcodes
     func stop() throws {
-        if (device == nil) {
+        if (device == nil || captureSession == nil) {
             throw MobileScannerError.alreadyStopped
         }
-        captureSession.stopRunning()
-        for input in captureSession.inputs {
-            captureSession.removeInput(input)
+        
+        captureSession!.stopRunning()
+        for input in captureSession!.inputs {
+            captureSession!.removeInput(input)
         }
-        for output in captureSession.outputs {
-            captureSession.removeOutput(output)
+        for output in captureSession!.outputs {
+            captureSession!.removeOutput(output)
         }
 
         latestBuffer = nil

@@ -15,7 +15,7 @@ public class MobileScannerPlugin: NSObject, FlutterPlugin, FlutterStreamHandler,
     var textureId: Int64!
 
     // Capture session of the camera
-    var captureSession: AVCaptureSession!
+    var captureSession: AVCaptureSession?
 
     // The selected camera
     weak var device: AVCaptureDevice!
@@ -239,7 +239,7 @@ public class MobileScannerPlugin: NSObject, FlutterPlugin, FlutterStreamHandler,
     }
 
     func start(_ call: FlutterMethodCall, _ result: @escaping FlutterResult) {
-        if (device != nil) {
+        if (device != nil || captureSession != nil) {
             result(FlutterError(code: "MobileScanner",
                                 message: "Called start() while already started!",
                                 details: nil))
@@ -289,17 +289,17 @@ public class MobileScannerPlugin: NSObject, FlutterPlugin, FlutterStreamHandler,
         }
         
         device.addObserver(self, forKeyPath: #keyPath(AVCaptureDevice.torchMode), options: .new, context: nil)
-        captureSession.beginConfiguration()
+        captureSession!.beginConfiguration()
         
         // Add device input
         do {
             let input = try AVCaptureDeviceInput(device: device)
-            captureSession.addInput(input)
+            captureSession!.addInput(input)
         } catch {
             result(FlutterError(code: "MobileScanner", message: error.localizedDescription, details: nil))
             return
         }
-        captureSession.sessionPreset = AVCaptureSession.Preset.photo
+        captureSession!.sessionPreset = AVCaptureSession.Preset.photo
         // Add video output.
         let videoOutput = AVCaptureVideoDataOutput()
         
@@ -307,15 +307,15 @@ public class MobileScannerPlugin: NSObject, FlutterPlugin, FlutterStreamHandler,
         videoOutput.alwaysDiscardsLateVideoFrames = true
         
         videoOutput.setSampleBufferDelegate(self, queue: DispatchQueue.main)
-        captureSession.addOutput(videoOutput)
+        captureSession!.addOutput(videoOutput)
         for connection in videoOutput.connections {
             // connection.videoOrientation = .portrait
             if position == .front && connection.isVideoMirroringSupported {
                 connection.isVideoMirrored = true
             }
         }
-        captureSession.commitConfiguration()
-        captureSession.startRunning()
+        captureSession!.commitConfiguration()
+        captureSession!.startRunning()
         let dimensions = CMVideoFormatDescriptionGetDimensions(device.activeFormat.formatDescription)
         let size = ["width": Double(dimensions.width), "height": Double(dimensions.height)]
         let answer: [String : Any?] = ["textureId": textureId, "size": size, "torchable": device.hasTorch]
@@ -374,17 +374,17 @@ public class MobileScannerPlugin: NSObject, FlutterPlugin, FlutterStreamHandler,
     //    }
 
     func stop(_ result: FlutterResult) {
-        if (device == nil) {
+        if (device == nil || captureSession == nil) {
             result(nil)
 
             return
         }
-        captureSession.stopRunning()
-        for input in captureSession.inputs {
-            captureSession.removeInput(input)
+        captureSession!.stopRunning()
+        for input in captureSession!.inputs {
+            captureSession!.removeInput(input)
         }
-        for output in captureSession.outputs {
-            captureSession.removeOutput(output)
+        for output in captureSession!.outputs {
+            captureSession!.removeOutput(output)
         }
         device.removeObserver(self, forKeyPath: #keyPath(AVCaptureDevice.torchMode))
         registry.unregisterTexture(textureId)

 name: mobile_scanner
 description: A universal barcode and QR code scanner for Flutter based on MLKit. Uses CameraX on Android, AVFoundation on iOS and Apple Vision & AVFoundation on macOS.
-version: 4.0.0
+version: 4.0.1
 repository: https://github.com/juliansteenbakker/mobile_scanner
 
 screenshots: