No available audioEngine input channels when running on hardware

Question

I'm trying to setup a tap on my audioEngine's inputNode for Speech Recognition. The following code works fine in the simulator, but not when I run it on my iPhone X running iOS 13.1.2.

Here is the relevant code:

if (audioEngine == nil || (!audioEngine!.isRunning)) {
    print("Setting up audioEngine")
    if audioEngine == nil {
        audioEngine = AVAudioEngine.init()
    }

    if(audioEngine!.inputNode.inputFormat(forBus: 0).channelCount == 0){
        print("Not enough available inputs!")
    }

    let recognitionFormat = audioEngine!.inputNode.inputFormat(forBus: 0)
    audioEngine!.inputNode.installTap(onBus: 0, bufferSize: 1024, format: recognitionFormat) {
        (buffer: AVAudioPCMBuffer, when: AVAudioTime) in
        self.recognitionRequest?.append(buffer)
    }
    audioEngine?.prepare()
    try audioEngine?.start()
}

I got no errors running in the simulator and Speech Recognition works fine. However, I got the following when running on my code:

Setting up audioEngine
[aurioc] AURemoteIO.cpp:1086:Initialize: failed: -10851 (enable 1, outf< 2 ch,      0 Hz, Float32, non-inter> inf< 2 ch,      0 Hz, Float32, non-inter>)
[aurioc] AURemoteIO.cpp:1086:Initialize: failed: -10851 (enable 1, outf< 2 ch,      0 Hz, Float32, non-inter> inf< 2 ch,      0 Hz, Float32, non-inter>)
Not enough available inputs!

If I don't return after channelCount == 0, the app will crash with:

[avae]            AVAEInternal.h:76    required condition is false: [AVAEGraphNode.mm:823:CreateRecordingTap: (IsFormatSampleRateAndChannelCountValid(format))]
*** Terminating app due to uncaught exception 'com.apple.coreaudio.avfaudio', reason: 'required condition is false: IsFormatSampleRateAndChannelCountValid(format)'

My Audio Session is setup like this:

try audioSession.setCategory(.playAndRecord, options: [.defaultToSpeaker])
try audioSession.setActive(true)

And a check of the available inputs yields:

LIST OF INPUTS: [<AVAudioSessionPortDescription: 0x280eac6b0, type = MicrophoneBuiltIn; name = iPhone Microphone; UID = Built-In Microphone; selectedDataSource = Bottom>]

I'm stumped. Any help?

Solution

I figured out the problem. The problem has to do with Flutter (which this app is using, but I didn't think was relevant.) It turns out that it is relevant. Apparently the FlutterAppDelegate@application method is not run on the main queue, which is where I was starting my AudioSession. When I started my AudioSession like this, my problem went away:

DispatchQueue.main.async {
    let audioSession = AVAudioSession.sharedInstance()
    do {
        try audioSession.setCategory(.playAndRecord, mode: .spokenAudio, options: .defaultToSpeaker)
        try audioSession.setActive(true, options: .notifyOthersOnDeactivation)
    } catch {
        print("Failed to setup audio session")
    }
}