Fix crash if either the Sheet or the SheetView got destroyed without letting the View know about it.
This commit makes sure we proper remove the Sheet and it's associated SheetView from the sheetViews-QHash the View holds if the Sheet is deleted. Also it makes sure that if the SheetView is deleted then we proper set it to NULL so we do not deal with dangling pointers what would lead to a crash latest if the View itself is destroyed.