Fixed #1428, an issue with preview the (empty) project placeholder.

The code treats this placeholder the same as any other proejct and attempts to open it, leading to a parse error.

gui.js

@@ -5440,17 +5440,20 @@ ProjectDialogMorph.prototype.setSource = function (source) {
             if (myself.task === 'open') {
 
                 src = localStorage['-snap-project-' + item.name];
+
+                if (src) {
                     xml = myself.ide.serializer.parse(src);
 
                     myself.notesText.text = xml.childNamed('notes').contents
                         || '';
                     myself.notesText.drawNew();
                     myself.notesField.contents.adjustBounds();
-                myself.preview.texture = xml.childNamed('thumbnail').contents
-                    || null;
+                    myself.preview.texture =
+                        xml.childNamed('thumbnail').contents || null;
                     myself.preview.cachedTexture = null;
                     myself.preview.drawNew();
                 }
+            }
             myself.edit();
         };
     } else { // 'examples'; 'cloud' is initialized elsewhere