fix release mode build

Author: lemz90

ios/Classes/Modules/ReaderModule.swift

@@ -1,18 +1,20 @@
 import Flutter
 import SquareMobilePaymentsSDK
+#if DEBUG
 import MockReaderUI
+#endif
 
 public class ReaderModule {
 
-
+    #if DEBUG
     static var mockReader: MockReaderUI? = {
         do {
             return try MockReaderUI(for: MobilePaymentsSDK.shared)
         } catch {
             return nil
         }
     }()
-
+    #endif
 
     static func parseTapToPayError(error: NSError, defaultError: String) -> String {
         let tapToPayReaderError = TapToPayReaderError(rawValue: error.code)
@@ -53,17 +55,22 @@ public class ReaderModule {
     }
 
     public static func showMockReaderUI(result: @escaping FlutterResult) {
+        #if DEBUG
         do {
             try mockReader?.present()
             result("Mock Reader has been successfully presented.")
         } catch let error {
             result(FlutterError(code: "SHOW_MOCK_READER_UI", message: error.localizedDescription, details: nil))
         }
+        #else
+        result(FlutterError(code: "SHOW_MOCK_READER_UI", message: "Mock Reader UI is only available in debug builds", details: nil))
+        #endif
     }
 
     public static func hideMockReaderUI(result: @escaping FlutterResult) {
-        
+        #if DEBUG
         mockReader?.dismiss()
+        #endif
         result("Mock Reader has been successfully hidden.")
     }