Eliminate random crash of theme engine

The WebEngine sites module now uses heavily fragment with models attached. The fragments are initialized through specific ActionFragments. After this implementation is in place, the applications starts to crash randomly when in sites view. There is no certain sequence of steps to reproduce, and once happening it doesn't happen again doing a refresh for instance. Doing the same thing again it doesn't yield the exception. And it appears after various actions (couldn't say it is a single widget affected).