fixes checkEmail error

lib/services/user_permission.dart

@@ -76,7 +76,9 @@ class UserPermissionApi {
     }
   }
 
-  Future<String?> checkEmail(email) async {
+
+
+  Future<String?> checkEmail(String email) async {
     try {
       final response = await _httpService.post(
         path: ApiEndpoints.checkEmail,
@@ -84,16 +86,26 @@ class UserPermissionApi {
         body: {"email": email},
         expectedResponseModel: (json) {
           if (json['statusCode'] != 400) {
-            return json["message"];
+            var message = json["message"];
+            if (message is String) {
+              return message;
+            } else {
+              return 'Unexpected message format';
+            }
           }
+          return null;
         },
       );
-      return response ?? [];
+      return response ?? 'Unknown error occurred';
     } on DioException catch (e) {
       if (e.response != null) {
+        print(e.response?.data['error']);
         final errorMessage = e.response?.data['error'];
-        return errorMessage;
+        return errorMessage is String
+            ? errorMessage
+            : 'Error occurred while checking email';
       }
+      return 'Error occurred while checking email';
     } catch (e) {
       return e.toString();
     }