Update cryptography_flutter/lib/src/flutter/flutter_ecdh.dart

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: terrier989

cryptography_flutter/lib/src/flutter/flutter_ecdh.dart

@@ -58,7 +58,7 @@ class FlutterEcdh extends Ecdh implements PlatformCryptographicAlgorithm {
 
   @override
   bool get isSupportedPlatform => isAndroid || isCupertino;
-
+  bool get isSupportedPlatform => FlutterCryptography.isPluginPresent && (isAndroid || isCupertino);
   String get _curveName {
     switch (keyPairType) {
       case KeyPairType.p256: