problem with change a port in arduino connection in apps design

i have some problem, i succeed to connection arduino with matlab menu portlist in connection. i try to change hardware connection to other port and refresh an error message appears "StringClient transport object must be connected using transport CONNECT". why its happen ? thanks

this my code

function startupFcn(app)
            spl = serialportlist('available');
            if isempty(spl)
                error('no serial ports available')
            end
                app.Property = serialport(spl(1),9600,"Timeout",5)
        end

this the error message

        function startButtonPushed(app, event)
            app.ConnLamp.Color = app.GREEN_COLOR;
            
            global val_stop val_data  
            val_stop 		= 0;
            val_sample 	        = 300;
            val_voltage 	= 0;
            val_count 	        = 1;
            while val_count <= val_sample
	        if val_stop == 1
		  break;
	        else
           	  ADC_value = str2double (readline(app.Property));
		  val_const(val_count) 	        = 5;
		  val_voltage(val_count) 	= ADC_value(1)*5/308;
		  plot(app.UIAxes,val_voltage);
		  app.UIAxes.XLim		= [0 val_count+50];
		  app.UIAxes.YLim		= [0 5.1];
                  
                  val_time(val_count) = val_count;

                  val_data = [val_voltage; val_time];
                  app.UITable.Data =val_data;

                  val_count = val_count + 1;
                  
 	       end
            end

this a refresh code

        function RefreshButtonPushed(app, event)
                delete(instrfind); 
                p = instrhwinfo('serial');
                app.PortList.Items = p.AvailableSerialPorts;
        end

this the pict of connection