Picker in SwiftUI Form turns gray after selecting a value

问题

For some reason the picker in my SwiftUI Form turns gray after I select a value.

My code:

Form {
  Section {
    TextField("title", text: $title)
      Picker(selection: $category, label: Text("category")) {
        ForEach(0..<categories.count) { index in
            Text(categories[index]).tag(index)
      }
    }
  }
}

来源：https://stackoverflow.com/questions/65405759/picker-in-swiftui-form-turns-gray-after-selecting-a-value