Fixing Form#row crash when returning from a block

This PR fixes a crash noted in issue #153.

It also fixes in issue where row equality comparisons are returning _false_ even though the row reference is exactly the same. The issue only occurs when subform checkboxes are swapped for another subform with checkboxes and the form is reloaded. 

The issue a bit enigmatic and it isn't clear to me _why_ it happens. The isEqual implementation on the row looks sounds and appears to work most of the time.

The patch is small and does fix the issue. However, the underlying cause is still in question. :-(