refresh _labelController after _handleControllerChanged

[dumabg]

If the SelectFromField text value is changed by code with a controller, it doesn't reflect the new value in the TextField. This happens because _labelController isn't updated.
_labelController must updated on _handleControllerChanged().

[hslbetto]

Sorry, I didan't get the specific use case of this issue.
For me, when i try to change the value of the select_form_field using controller or initialvalue, it work fine
The same job of your commit is did on didUpdateWidget method.
Now the job is duplicated. Unless that have some specific use case.