Error Multitrack Audio AVAudioEngine SwiftUI

Hi, I'm trying to create a multitrack audio Player with AVAudioEngine in SwiftUI. It's basically 4 individual tracks (drums, bass, keys, click) playing at the same time. I've tried to build it using documentation I've found on google, but I'm running into the following error: "Terminating app due to uncaught exception 'com.apple.coreaudio.avfaudio', reason: 'required condition is false: [_nodes containsObject: node1] && [_nodes containsObject: node2]'" - can you please help me?

As I'm new to coding, don't hesitate to let me know would be able to play sound.

import SwiftUI
import AVFoundation

class AudioMixer {
  let engine = AVAudioEngine()
  let player = AVAudioPlayer()
  let drumPlayer = AVAudioPlayerNode()
  let bassPlayer = AVAudioPlayerNode()
  let keysPlayer = AVAudioPlayerNode()
  let clickPlayer = AVAudioPlayerNode()
  let mixer = AVAudioMixerNode()


init() {
  let audioSession = AVAudioSession.sharedInstance()
  do {
  try audioSession.setCategory(AVAudioSession.Category.playback, options: .mixWithOthers)
  try audioSession.setActive(true)
 } catch {
  print(error.localizedDescription)
 }




if let drumAudioPath = Bundle.main.path(forResource: "Drums1", ofType: "mp3") {
  let drumsAudioUrl = URL(fileURLWithPath: drumAudioPath)
  let drumsAudioFile = try? AVAudioFile(forReading: drumsAudioUrl)
  engine.connect(drumPlayer, to: mixer, format: drumsAudioFile?.processingFormat)
  drumPlayer.scheduleFile(drumsAudioFile!, at: nil, completionHandler: nil)
}
drumPlayer.play()
engine.attach(drumPlayer)


if let bassAudioPath = Bundle.main.path(forResource: "Bass1", ofType: "mp3") {
  let bassAudioUrl = URL(fileURLWithPath: bassAudioPath)
  let bassAudioFile = try? AVAudioFile(forReading: bassAudioUrl)
  engine.connect(bassPlayer, to: mixer, format: bassAudioFile?.processingFormat)
  bassPlayer.scheduleFile(bassAudioFile!, at: nil, completionHandler: nil)
  }
bassPlayer.play()
engine.attach(bassPlayer)


 if let keysAudioPath = Bundle.main.path(forResource: "Keys1", ofType: "mp3") {
  let keysAudioUrl = URL(fileURLWithPath: keysAudioPath)
  let keysAudioFile = try? AVAudioFile(forReading: keysAudioUrl)
  engine.connect(keysPlayer, to: mixer, format: keysAudioFile?.processingFormat)
  keysPlayer.scheduleFile(keysAudioFile!, at: nil, completionHandler: nil)
  }
keysPlayer.play()
engine.attach(keysPlayer)


 if let clickAudioPath = Bundle.main.path(forResource: "Click1", ofType: "mp3") {
  let clickAudioUrl = URL(fileURLWithPath: clickAudioPath)
  let clickAudioFile = try? AVAudioFile(forReading: clickAudioUrl)
  engine.connect(clickPlayer, to: mixer, format: clickAudioFile?.processingFormat)
  clickPlayer.scheduleFile(clickAudioFile!, at: nil, completionHandler: nil)
  }
clickPlayer.play()
engine.attach(clickPlayer)

engine.attach(mixer)

engine.prepare()
  do {
  try engine.start()
  }
  catch {
  print(error.localizedDescription)
  }
}