"Split UTXO" checkbox not working in xsn-qt

## Description
In "Send XSN" section, clicking on `Split UTXO` checkbox doesn't work.
`# of outputs` field stays inactive (non-fillable).

## Expected behaviour
`Split UTXO` checkbox should be "tickable".
Enabling `Split UTXO` should activate the `# of outputs` field.

## XSN Core version
xsn-qt v1.0.21-7fe0dae
All OS (Windows, Linux, macOS)

