When starting a wiki on a sub wiki page, main wiki main page is broken by statistics module

There is two issue:

• when the class reference is null in a BaseObject, getXClass return class on Main.WebHome document
• there is a desynchronization of xcontext and ExectionContext xcontext in statistics module that makes loading sub wiki main page as main wiki main page in the cache storage

To reproduce:

• enable statistics
• start xwiki
• go to a subwiki page
• go to main wiki main page -> it's the subwiki main page