Cannot convert value of type 'String' to type 'NSNumber' in coercion

Code Block 
import SwiftUI
struct TemperatureView: View {
    
    @State private var inputValue = ""
    
     let inputUnits = [
        "celsius [°C]",
        "kelvin [K]",
        "fahrenheit [°F]"
    ]
    let outputUnits = [
        "celsius [°C]",
        "kelvin [K]",
        "fahrenheit [°F]"
   ]
    @State private var inputUnitValue = 0
    
    @State private var outputUnitValue = 1
    
    
    var after: String{
        var input: Measurement<UnitTemperature>
        var output: String = ""
        
        switch inputUnits[inputUnitValue] {
        case "celsius [°C]": input = Measurement(value: Double(inputValue) ?? 0, unit: .celsius)
        case "kelvin [K]": input = Measurement(value: Double(inputValue) ?? 0, unit: .kelvin)
        case "fahrenheit [°F]": input = Measurement(value: Double(inputValue) ?? 0, unit: .fahrenheit)
        default: input = Measurement(value: Double(inputValue) ?? 0, unit: UnitTemperature.celsius)
        }
        switch outputUnits[outputUnitValue] {
        case "celsius [°C]": output = outputFormatter.string(from: input.converted(to: .celsius))
        case "kelvin [K]": output = outputFormatter.string(from: input.converted(to: .kelvin))
        case "fahrenheit [°F]": output = outputFormatter.string(from: input.converted(to: .fahrenheit))
        default: output = String(describing: input.converted(to: UnitTemperature.celsius))
            
        }
            
        return output
    }
    
    @Environment(\.presentationMode) var presentationMode
    let outputFormatter: MeasurementFormatter = {
            let nf = NumberFormatter()
            nf.locale = Locale.current
            nf.usesSignificantDigits = true
            let mf = MeasurementFormatter()
            mf.numberFormatter = nf
            mf.unitOptions = .providedUnit
            return mf
        }()
        
    var body: some View {
        NavigationView{
            Form{
                Section(header: Text("Enter your Input value")){
                        TextField("Have a goal?", text: $inputValue)
                            .keyboardType(.decimalPad)
                }
                
                Section(header: Text("Input")){
                    Picker("Input values", selection: $inputUnitValue){
                        ForEach(0..<inputUnits.count){ item in
                            Text(inputUnits[item])
                        }
                    }
                }
                
                Section(header: Text("Output")){
                    Picker("Output values", selection: $outputUnitValue){
                        ForEach(0..<outputUnits.count){ item in
                            Text(outputUnits[item])
                        }
                    }
                }
                
                Section(header: Text("Check your Output value")){
                    Text("\(after as NSNumber, formatter: outputFormatter)")
                }
                
            }
            .navigationBarTitle("Temperature")
            .navigationBarItems(trailing:
                                    Button(action: {
                                        presentationMode.wrappedValue.dismiss()
                                    }) {
                                        Image(systemName: "xmark").font(.title).foregroundColor(.blue)
                                    }
            )
        }
    }
    
}