Handle null value.

framework/src/Volo.Abp.AspNetCore.Mvc/Volo/Abp/AspNetCore/Mvc/AspNetCoreApiDescriptionModelProviderOptions.cs

@@ -63,14 +63,14 @@ namespace Volo.Abp.AspNetCore.Mvc
                 if (apiParameterDescription.ModelMetadata is DefaultModelMetadata defaultModelMetadata)
                 {
                     var jsonPropertyNameAttribute = (System.Text.Json.Serialization.JsonPropertyNameAttribute)
-                        defaultModelMetadata.Attributes.PropertyAttributes.FirstOrDefault(x => x is System.Text.Json.Serialization.JsonPropertyNameAttribute);
+                        defaultModelMetadata?.Attributes?.PropertyAttributes?.FirstOrDefault(x => x is System.Text.Json.Serialization.JsonPropertyNameAttribute);
                     if (jsonPropertyNameAttribute != null)
                     {
                         return jsonPropertyNameAttribute.Name;
                     }
 
                     var jsonPropertyAttribute = (Newtonsoft.Json.JsonPropertyAttribute)
-                        defaultModelMetadata.Attributes.PropertyAttributes.FirstOrDefault(x => x is Newtonsoft.Json.JsonPropertyAttribute);
+                        defaultModelMetadata?.Attributes?.PropertyAttributes?.FirstOrDefault(x => x is Newtonsoft.Json.JsonPropertyAttribute);
                     if (jsonPropertyAttribute != null)
                     {
                         return jsonPropertyAttribute.PropertyName;