Swift - Asking for microphone permission

Question

Within my swift app, this is the code I have to request for microphone and camera permissions.

class func requestCameraPermission(completionBlock:(Bool throws -> Void)) rethrows {
    if (AVCaptureDevice.respondsToSelector(Selector("requestAccessForMediaType:completionHandler"))) {
        AVCaptureDevice.requestAccessForMediaType(AVMediaTypeVideo, completionHandler: {
            granted in
            dispatch_async(dispatch_get_main_queue()) {
                try! completionBlock(granted)
            }
        })
    } else {
        try! completionBlock(true)
    }
}


class func requestMicrophonePermission(completionBlock:(Bool throws -> Void)) rethrows {
    if AVAudioSession.sharedInstance().respondsToSelector("requestRecordPermission:") {
        AVAudioSession.sharedInstance().requestRecordPermission {
            granted in
            dispatch_async(dispatch_get_main_queue()) {
                try! completionBlock(granted)
            }
        }
    }
}

And this is the code I have to attach the camera and show an error if some permissions are missing.

func attachCamera() {
    do {
        try self.camera.start()
    } catch BESwiftCameraErrorCode.CameraPermission {
        self.showCameraPermissionAlert()
    } catch BESwiftCameraErrorCode.MicrophonePermission {
        self.showMicrophonePermissionAlert()
        print("mic error!!")
    } catch {
        self.showUnknownErrorAlert()
    }
}

I have tried not allowing for the camera permission. I see an alert box saying that I need camera permissions. The app does not crash.

But when I try not allowing the microphone permission, the app crashes.

With this line highlighted in the crash log:

try! completionBlock(granted)

And this error:

fatal error: 'try!' expression unexpectedly raised an error: TestApp.CameraErrorCode.MicrophonePermission: file /Library/Caches/com.apple.xbs/Sources/swiftlang_PONDEROSA/swiftlang_PONDEROSA-700.1.101.6/src/swift/stdlib/public/core/ErrorType.swift, line 50

What is causing this? How do I get the microphone permission alert to show instead of the app crashing?