chore: improve handling for nil data and null values in transaction error paths

Author: dporwal-shipit

scgateway/android/src/main/kotlin/com/example/scgateway_flutter_plugin/ScgatewayFlutterPluginTxnExt.kt

@@ -77,6 +77,7 @@ fun ScgatewayFlutterPlugin.txnOnSuccessCallback(transactionResult: TransactionRe
 
     } catch (e: Exception) {
         e.printStackTrace()
+        result.error("TXN_ERROR", e.message ?: "Unknown error in transaction callback", null)
     }
 }
 

scgateway/ios/Classes/SwiftScgatewayFlutterPlugin.swift

@@ -404,7 +404,7 @@ public class SwiftScgatewayFlutterPlugin: NSObject, FlutterPlugin, FlutterStream
                                         resDict["transactionId"] = transactionId
                                         resDict["signup"] = signup
 
-                                        let jsonData = try! JSONSerialization.data(withJSONObject: resDict, options: [])
+                                        let jsonData = try JSONSerialization.data(withJSONObject: resDict, options: [])
                                         let jsonString = String(data: jsonData, encoding: .utf8)
 
                                         result(jsonString)
@@ -413,7 +413,7 @@ public class SwiftScgatewayFlutterPlugin: NSObject, FlutterPlugin, FlutterStream
                                     }
 
                                 } catch {
-                                    /// Catch exception
+                                    result(FlutterError(code: "IMR_SETUP_ERROR", message: error.localizedDescription, details: nil))
                                 }
 
                             default: