[Electric-Coin-Company#1420] Synchronizer.proposeShielding crashes wh…

…en there are insufficient funds to shield

- Check for the null value of the pointer when constructing propose shielding

[Electric-Coin-Company#1420] Synchronizer.proposeShielding crashes when there are insufficient funds to shield

- throw an error instead of returning nil value

[Electric-Coin-Company#1420] Synchronizer.proposeShielding crashes when there are insufficient funds to shield

- Changelog updated
- Error message polished a bit

CHANGELOG.md

@@ -6,6 +6,9 @@ and this library adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+## Fixed
+- The backend method proposeShielding checks the pointer for a null value before attempting to construct the Data with it. Without this check, proposeShielding would crash when there were either zero funds to shield or when the amount was less than the threshold defined by the client.
+
 # 2.1.5 - 2024-04-18
 
 ## Changed

Sources/ZcashLightClientKit/Rust/ZcashRustBackend.swift

@@ -752,11 +752,15 @@ struct ZcashRustBackend: ZcashRustBackendWelding {
         )
 
         guard let proposal else {
-            throw ZcashError.rustShieldFunds(lastErrorMessage(fallback: "`proposeShielding` failed with unknown error"))
+            throw ZcashError.rustShieldFunds(lastErrorMessage(fallback: "Failed with nil proposal."))
         }
 
         defer { zcashlc_free_boxed_slice(proposal) }
-
+
+        guard proposal.pointee.ptr != nil else {
+            throw ZcashError.rustShieldFunds(lastErrorMessage(fallback: "Insufficient funds to shield."))
+        }
+
         return try FfiProposal(contiguousBytes: Data(
             bytes: proposal.pointee.ptr,
             count: Int(proposal.pointee.len)