CreateFile with uri causes methids to fail

[Al4He6]

After CreateFile with a uri,
then methods give error
"Cannot read prooerties of null"
(choose a file, then touch Short button)

function OnStart()
{
	//Create a layout with objects vertically centered.
	lay = app.CreateLayout( "Linear", "VCenter,FillXY" )
	
  app.ChooseFile( "Choose a file", "*/*", test )
 
  txt = app.CreateText( "" )
  lay.AddChild( txt )

	//Create a text label and add it to layout.
	btnshort = app.CreateButton( "Long" )
	btnshort.SetOnTouch( btnshort_OnTouch )
	lay.AddChild( btnshort )
	
	btnlong = app.CreateButton( "Short" )
	btnlong.SetOnTouch( btnlong_OnTouch )
	lay.AddChild( btnlong )
	
	//Add layout to app.	
	app.AddLayout( lay )
}

function test( file )
{
alert(file)
  fileuri = file;
	txt.SetText( app.ReadFile( file ) )
}

function btnshort_OnTouch()
{
	app.WriteFile( fileuri, "Hello how are you?" )
	txt.SetText( app.ReadFile( fileuri ) )
}

function btnlong_OnTouch()
{
//	app.WriteFile( fileuri, "I'm good." )
  var f = app.CreateFile( fileuri,"w")
  f.WriteData("I'm good.","Text")
  f.Close()
  txt.SetText( app.ReadFile( fileuri ) )
}

```file