iOS app crashes when running this code

This code crashed when running startApplianceListener() when using onAppear() on a swiftui view

import AVFAudio
import CoreML
import SoundAnalysis

/// An observer that receives results from a classify sound request.
class ResultsObserver: NSObject, SNResultsObserving {
    /// Notifies the observer when a request generates a prediction.
    func request(_ request: SNRequest, didProduce result: SNResult) {
        // Downcast the result to a classification result.
        guard let result = result as? SNClassificationResult else  { return }

        // Get the prediction with the highest confidence.
        guard let classification = result.classifications.first else { return }

        // Get the starting time.
        let timeInSeconds = result.timeRange.start.seconds

        // Convert the time to a human-readable string.
        let formattedTime = String(format: "%.2f", timeInSeconds)
        print("Analysis result for audio at time: \(formattedTime)")

        // Convert the confidence to a percentage string.
        let percent = classification.confidence * 100.0
        let percentString = String(format: "%.2f%%", percent)

        // Print the classification's name (label) with its confidence.
        print("\(classification.identifier): \(percentString) confidence.\n")
    }

    /// Notifies the observer when a request generates an error.
    func request(_ request: SNRequest, didFailWithError error: Error) {
        print("The the analysis failed: \(error.localizedDescription)")
    }

    /// Notifies the observer when a request is complete.
    func requestDidComplete(_ request: SNRequest) {
        print("The request completed successfully!")
    }
}

func startApplianceListener() {
    do {
        var audioEngine: AVAudioEngine?
        var inputBus: Int?
        var inputFormat: AVAudioFormat?
        var streamAnalyzer: SNAudioStreamAnalyzer

        func startAudioEngine() {
            audioEngine = AVAudioEngine()
            
            inputBus = AVAudioNodeBus(0)
            inputFormat = audioEngine!.inputNode.inputFormat(forBus: inputBus!)

            do {
                try audioEngine!.start()
            } catch {
                print("Unable to start AVAudioEngine \(error.localizedDescription)")
                exit(1)
            }
        }

        startAudioEngine()

        streamAnalyzer = SNAudioStreamAnalyzer(format: inputFormat!)

        let version1 = SNClassifierIdentifier.version1
        let request = try SNClassifySoundRequest(classifierIdentifier: version1)

        let resultsObserver = ResultsObserver()

        try streamAnalyzer.add(request,
                               withObserver: resultsObserver)

        func installAudioTap() {
            audioEngine!.inputNode.installTap(onBus: inputBus!,
                                              bufferSize: 8192,
                                              format: inputFormat,
                                              block: analyzeAudio(buffer:at:))
        }

        let analysisQueue = DispatchQueue(label: "dev.paulwalker.AnalysisQueue")

        func analyzeAudio(buffer: AVAudioBuffer, at time: AVAudioTime) {
            analysisQueue.async {
                streamAnalyzer.analyze(buffer,
                                       atAudioFramePosition: time.sampleTime)
            }
        }

        installAudioTap()
    } catch {}
}