show message when cipher not exist

shadowsocks-csharp/Data/i18n.csv

@@ -76,6 +76,7 @@ New server,Новый сервер,未配置的服务器,新伺服器,新規サ
 Move &Up,Выше,上移(&U),上移 (&U),上に移動 (&U)
 Move D&own,Ниже,下移(&O),下移 (&O),下に移動 (&O)
 deprecated,Устаревшее,不推荐,不推薦,非推奨
+"Encryption method {0} not exist, will replace with {1}",,"加密方法{0}不存在，将使用{1}代替",,
 ,,,,
 #Statistics Config,,,,
 ,,,,

shadowsocks-csharp/View/ConfigForm.cs

@@ -79,7 +79,14 @@ namespace Shadowsocks.View
             public static EncryptionMethod GetMethod(string name)
             {
                 if (!init) Init();
-                return methodByName[name];
+                bool success = methodByName.TryGetValue(name, out EncryptionMethod method);
+                if (!success)
+                {
+                    string defaultMethod = Server.DefaultMethod;
+                    MessageBox.Show(I18N.GetString("Encryption method {0} not exist, will replace with {1}", name, defaultMethod), I18N.GetString("Shadowsocks"));
+                    return methodByName[defaultMethod];
+                }
+                return method;
             }
 
             private EncryptionMethod(string name, bool deprecated)