page.selected property is not work
sketch version is 52.6

I want to get current page's selected artboards,  the code is:
```
          const document = this.document;
          const allArtboards = [];
          const selectedArtboards = [];
          const pages = document.pages();
          for (var i = 0, len = pages.count(); i < len; ++i) {
            var page = pages[i];
            var data = {
              name: "".concat(page.name()),
              objectID: "".concat(page.objectID()),
              artboards: []
            };
            console.log(`page data is ${JSON.stringify(data)}`);
            console.log(`page select is ${page.selected}`);
            console.log(`page select is ${page.isSelected()}`);
            if (data.name != 'Symbols' && page.selected) {
              var artboards = page.artboards();
              var count = artboards.count();
      
              if (count > 0) {
                for (var _i = 0, _len = count; _i < _len; ++_i) {
                  var artboard = artboards[_i];
                  data.artboards.push({
                    name: "".concat(artboard.name()),
                    objectID: "".concat(artboard.objectID()),
                    MSArtboardGroup: artboard
                  });
                }
      
                selectedArtboards.push(data);
              }
            }
          }
```
but console info is:
```
page data is {"name":"Page 1","objectID":"ADEF3094-BE41-4D8C-AD91-1B5A7EC006D8","artboards":[]}
handlers
page select is undefined
handlers
page select is 0
handlers
page data is {"name":"Symbols","objectID":"66688179-D8A8-4E44-AF5B-6B5DEC32F9D9","artboards":[]}
handlers
page select is undefined
handlers
page select is 0
```
`page.selected` value is `undefined`