Merge pull request #713 from microsoft/fix/NRE-on-base-type

fix: a bug where checking the whether a type is referenced would lead to a null reference exception

Author: baywet

src/Microsoft.OpenApi.OData.Reader/Common/Utils.cs

@@ -238,7 +238,8 @@ internal static bool IsBaseTypeReferencedAsTypeInModel(
                 if (isReferencedInAction) return true;
 
                 // Recursively check the base type
-                return model.IsBaseTypeReferencedAsTypeInModel(baseType.BaseType, structuredTypes, actions);
+                if (baseType.BaseType is not null)
+                    return model.IsBaseTypeReferencedAsTypeInModel(baseType.BaseType, structuredTypes, actions);
             }
 
             return false;