Fix to map_row build_cell to return nil instead of value of swizzle which caused crash on becomeFirstResponder

The return value of `Formotion::RowType::MapRow#build_cell` was the value of the call to         `cell.swizzle(:layoutSubviews) do` (line 108) instead of nil. This was causing a crash with a method not found error when `becomeFirstResponder` was called during focus change. This change request ads a nil return value to stop the crash.