guard against a nil device in the background queue

@@ -214,18 +214,28 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
                 print("Failed to reset zoom scale")
             }
             
-            let dimensions = CMVideoFormatDescriptionGetDimensions(self.device.activeFormat.formatDescription)
+            if let device = self.device {
+                let dimensions = CMVideoFormatDescriptionGetDimensions(
+                    device.activeFormat.formatDescription)
+                let hasTorch = device.hasTorch
                 
                 DispatchQueue.main.async {
                     completion(
                         MobileScannerStartParameters(
                             width: Double(dimensions.height),
                             height: Double(dimensions.width),
-                        hasTorch: self.device.hasTorch,
-                        textureId: self.textureId
+                            hasTorch: hasTorch,
+                            textureId: self.textureId ?? 0
                         )
                     )
                 }
+                
+                return
+            }
+            
+            DispatchQueue.main.async {
+                completion(MobileScannerStartParameters())
+            }
         }
     }