windowPreview: window has no associated app when dragged to a new workspace

Drag a window to a new workspace such that it changes the number of workspaces. This causes a new WindowPreview to be created after the window has been removed from the window tracker, but before it has been added again. As a result tracker.get_window_app(this.metaWindow); returns none and trying to get the icon from it will fail and the preview will not show up on the new workspace.