Go back to previous page in Swift UI after Async func completion

I have a view with the following code

@Environment(\.presentationMode) var presentationMode
          
  ...

 Button {
    Task {
       await asyncFunction(data:data) {                            
          self.presentationMode.wrappedValue.dismiss()
       }
    }
 } label: {
   Text("Save")
 }

And my async function goes as follow:

func asyncFunction(data: Data, completion: @escaping () -> Void) async {
  
  // Prepare URL
  let url = URL(string: "api-url")
  guard let requestUrl = url else { fatalError() }

  // HTTP Request payload which will be sent in HTTP Request Body
  let d = try! JSONEncoder().encode(data)

  ...

  // Perform HTTP Request
  let (responseData, _) = try! await URLSession.shared.data(for: request)

  // Convert HTTP Response Data to a String
     if let responseData = Optional(responseData) {
         if let dataString = String(data: responseData, encoding: .utf8) {
             print("Response:\n \(dataString)")
         }
     }

  // Call the completion handler
  completion()
}

The problem is that, once the the request is complete and it's time to call the dismiss(), the app crashes with this error
Thread 18: EXC_BREAKPOINT (code=1, subcode=0x11022cd24).

Looks like a problem with the closure, but not sure to understand what is it?

Several issues

1. Floating Task like what you have in the Button are traditionally considered bad practice because you lose control of the Task itself.

2. Completion handlers are a thing of the past when adopting async await you should Never (rare exceptions I only know of one) go from async await to a completion handler.

If you have to go from a completion handler to async await you have to use a CheckedContinuation.

Since async await acts as if it is synchronous you can just remove the completion handler completely.

 Button {
    Task {
       await asyncFunction(data:data)
       self.presentationMode.wrappedValue.dismiss()

    }
 } label: {
   Text("Save")
 }

func asyncFunction(data: Data) async {
  
  // Prepare URL
  let url = URL(string: "api-url")
  guard let requestUrl = url else { fatalError() }

  // HTTP Request payload which will be sent in HTTP Request Body
  let d = try! JSONEncoder().encode(data)

  ...

  // Perform HTTP Request
  let (responseData, _) = try! await URLSession.shared.data(for: request)

  // Convert HTTP Response Data to a String
     if let responseData = Optional(responseData) {
         if let dataString = String(data: responseData, encoding: .utf8) {
             print("Response:\n \(dataString)")
         }
     }

  // NO Call the completion handler

}    

Now to get rid of the Task you should use .task

struct AsyncButton: View {
    @Environment(\.dismiss) var dismiss

    @State private var isProcessingFunction: Bool = false
    var body: some View {
        Button {
            isProcessingFunction.toggle()
        } label: {
          Text("Save")
        }.task(id: isProcessingFunction) {
            guard isProcessingFunction else {
                return
            }
            await asyncFunction(data:data)
            dismiss()
            isProcessingFunction = false
        }
    }
}

And with this you can take full advantage of async await

struct AsyncButton: View {
    @Environment(\.dismiss) var dismiss

    @State private var isProcessingFunction: Bool = false
    var body: some View {
        Button {
            isProcessingFunction.toggle()
        } label: {
          Text("Save")
                .overlay {
                    //Add a ProgressView
                    if isProcessingFunction {
                        ProgressView()
                    }
                }
        }
        .disabled(isProcessingFunction) //Maybe disable button to prevent duplicate calls. Or not to allow the user to cancel the call.
        .task(id: isProcessingFunction) {
            guard isProcessingFunction else {
                return
            }
            await asyncFunction(data:data)
            dismiss()
            isProcessingFunction = false
        }
    }
}