.m callback to app designer issue

When I call a .m file from the app designer, in this case a listener handle the callback from the .m file to the app instantiates the startupFcn and makes a new figure. Is there a way to avoid or suppress this? I can get around this by deleting the new figure, but now the app has a "flashing" feel every time the file under watch is updated

methods (Access = private)
        % Code that executes after component creation
        function startupFcn(app)
            global mylistener;
            global file;
            getcaller=dbstack();
            mycaller=getcaller(5).name;
            switch mycaller
                case {'eventhandlerChanged'}
                    temp=findall(0,'Type','figure');
                  delete(temp(2));
                case  {'AppManagementService.runAppWithTryCatch'}
                    filepath='C:\test';
                    file=System.IO.FileSystemWatcher(filepath);
                    file.Filter='November2018.txt';
                    file.EnableRaisingEvents=true;
                     mylistener=addlistener(file,'Changed',@eventhandlerChanged);
                    app.Label2.Text='NOT UPDATED';
                    app.Label.Text='';
                    app.Label3.Text='';
            end
            
        end

  function eventhandlerChanged(~,h1)
  MainFunction(app1,h1);
  end