Fix crash on update in background

[tomaz]

Without this, Row would crash during didEndDisplayCell handling if table view is not on active controller (for example if there's modal controller on top of table view controller).

The solution here is to simply ignore the call if didEndDisplayCell block is not assigned on Row. This preserves existing functionality for cases where block is used but prevents the crash otherwise.

Besided crash fix, I also added ability to delete section from source.