when you use data['payee(name)'] it will define data type as dynamic in case it can't find value it that key and will return null (in this case your key is "payee(name)") in this case you can define type for value that return from future like this

FutureBuilder<File>( // change File to type you want ex. model String etc.
  future: // your future must return value File like <File>,
  builder: (context, snapshot) {
 // return widget here
}