Gets the object instance passed as the source of the binding.
While a binding can outlive the source as the binding doesn't hold a strong reference to the source, the binding will try to finalize itself when it does not have a valid source. If the source, however, is destroyed before the binding is fully finalized then this function will return `NULL`.
| this |
a binding |
|
the source, or `NULL` if it doesn't exist anymore |