Gracefully handle attempted spend to a UA with neither transparent nor sapling recipients

zcash_client_backend/src/data_api/error.rs

@@ -54,6 +54,9 @@ pub enum Error<DataSourceError, CommitmentTreeError, SelectionError, FeeError, N
     /// It is forbidden to provide a memo when constructing a transparent output.
     MemoForbidden,
 
+    /// Attempted to create a spend to an unsupported output type (i.e. Orchard)
+    UnsupportedOutputType,
+
     /// A note being spent does not correspond to either the internal or external
     /// full viewing key for an account.
     NoteMismatch(NoteRef),
@@ -111,6 +114,7 @@ where
             Error::ScanRequired => write!(f, "Must scan blocks first"),
             Error::Builder(e) => write!(f, "An error occurred building the transaction: {}", e),
             Error::MemoForbidden => write!(f, "It is not possible to send a memo to a transparent address."),
+            Error::UnsupportedOutputType => write!(f, "Attempted to create spend to an unsupported output type"),
             Error::NoteMismatch(n) => write!(f, "A note being spent ({}) does not correspond to either the internal or external full viewing key for the provided spending key.", n),
 
             #[cfg(feature = "transparent-inputs")]

zcash_client_backend/src/data_api/wallet.rs

@@ -546,12 +546,26 @@ where
                     .memo
                     .as_ref()
                     .map_or_else(MemoBytes::empty, |m| m.clone());
-                builder.add_sapling_output(
+
-                    external_ovk,
+                if ua.sapling().is_some() {
-                    *ua.sapling().expect("TODO: Add Orchard support to builder"),
+                    builder.add_sapling_output(
-                    payment.amount,
+                        external_ovk,
-                    memo.clone(),
+                        *ua.sapling().unwrap(),
-                )?;
+                        payment.amount,
+                        memo.clone(),
+                    )?;
+                } else if ua.transparent().is_some() {
+                    if payment.memo.is_some() {
+                        return Err(Error::MemoForbidden);
+                    } else {
+                        builder.add_transparent_output(
+                            ua.transparent().unwrap(), 
+                            payment.amount
+                        )?;
+                    }
+                } else {
+                    return Err(Error::UnsupportedOutputType);
+                }
                 sapling_output_meta.push((
                     Recipient::Unified(ua.clone(), PoolType::Shielded(ShieldedProtocol::Sapling)),
                     payment.amount,