T1720
Right now the app is set to lock the UI on the Send and SendToken screens while the utxo set is refreshing. However, the user may still click off the Send or SendToken screen and then navigate back. This could allow the user to send a transaction with an invalid utxo set. The transaction won't actually send, they will get an error msg, but it's confusing and not great UX.
This diff locks the nav bar while the utxo set refreshes.