QmlDesigner: fix that an invalid root item refreshes the preview

Change-Id: I8d917dfd92261a88472bd49577731f5bcf468d49
Reviewed-by: Marco Bubke <marco.bubke@digia.com>

share/qtcreator/qml/qmlpuppet/qml2puppet/instances/quickitemnodeinstance.cpp

@@ -392,8 +392,15 @@ QImage QuickItemNodeInstance::renderPreviewImage(const QSize &previewImageSize)
 {
     QRectF previewItemBoundingRect = boundingRect();
 
-    if (previewItemBoundingRect.isValid() && quickItem())
-        return designerSupport()->renderImageForItem(quickItem(), previewItemBoundingRect, previewImageSize);
+    if (previewItemBoundingRect.isValid() && quickItem()) {
+        if (quickItem()->isVisible()) {
+            return designerSupport()->renderImageForItem(quickItem(), previewItemBoundingRect, previewImageSize);
+        } else {
+            QImage transparentImage(previewImageSize, QImage::Format_ARGB32_Premultiplied);
+            transparentImage.fill(Qt::transparent);
+            return transparentImage;
+        }
+    }
 
     return QImage();
 }