Flutter/Dart Scope Problem - Return Info Needed To Prevent Duplicate File Upload?

I've got a button which uploads a file to a server and returns the oldname of the file. Before uploading, I'd like to check if the file has already been uploaded. But unfortunately, there seems to be a problem with scope and the oldname variable always returns null even if the previous upload was successful. Here's the code;

    FlatButton(
            onPressed: () async {
             String oldname;
              if (oldname == null)  {
                   oldname = await uploadAudio(
                   currentuserid: currentuserid,
                   filepath: _current.path);                 
              } else {alertUpLoaded(context);}
            },        
          )

Your issue is that oldname will always be null since its initialization String oldname is within the same build method.

Move String oldname above the widget’s build function