OpenFile crashes on OSX

The following is a small program to capture a filename using ui.OpenFile:

``` go
package main

import (
"github.com/andlabs/ui"
)

var window ui.Window
var srcName string

func loadFile(filename string) {
srcName = filename
}

func initUI() {
b := ui.NewButton("Read")
b.OnClicked(func() {
ui.OpenFile (window, loadFile)
})
window = ui.NewWindow("Window", 200, 200, b)
window.OnClosing(func() bool {
ui.Stop()
return true
})
window.Show()
}

func main() {
go ui.Do(initUI)
err := ui.Go()
if err != nil {
panic(err)
}
}
```

When I run it on OSX, it crashes with the following stack trace (I don't have an available Windows or Unix box to check those):

```
$ sippui
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x2d pc=0x4041f43]

goroutine 1 [running, locked to thread]:
github.com/andlabs/ui.finishOpenFile(0x4358b30, 0xc20801d5a0)
/Users/Raul/gocode/src/github.com/andlabs/ui/dialog_darwin.go:24 +0xa3
github.com/andlabs/ui._Cfunc_uimsgloop()
/Users/Raul/gocode/src/github.com/andlabs/ui/:442 +0x45
github.com/andlabs/ui.uimsgloop()
/Users/Raul/gocode/src/github.com/andlabs/ui/uitask_darwin.go:27 +0x1b
github.com/andlabs/ui.Go(0x0, 0x0)
/Users/Raul/gocode/src/github.com/andlabs/ui/uitask.go:22 +0x74
main.main()
/Users/Raul/gocode/src/github.com/Causticity/sipp/sippui/main.go:29 +0x3b

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
/usr/local/go/src/runtime/asm_amd64.s:2232 +0x1

goroutine 6 [chan receive]:
github.com/andlabs/ui.uiissueloop()
/Users/Raul/gocode/src/github.com/andlabs/ui/uitask.go:59 +0x53
created by github.com/andlabs/ui.Go
/Users/Raul/gocode/src/github.com/andlabs/ui/uitask.go:21 +0x6d
$
```

To me it looks like OpenFile has a bug causing it to crash on invocation of the callback, but perhaps I'm missing something.