adds nullable checks.

Author: Andrew Omondi

src/Kiota.Builder/KiotaBuilder.cs

@@ -955,10 +955,10 @@ x.Parent is CodeIndexer ||
 
         Parallel.ForEach(unmappedRequestBuilderTypes, parallelOptions, x =>
         {
-            var parentClass = x.Parent?.Parent as CodeClass;
             var parentNS = x.Parent?.Parent?.Parent as CodeNamespace;
+            CodeClass[] exceptions = x.Parent?.Parent is CodeClass parentClass ? [parentClass] : [];
             x.TypeDefinition = parentNS?.FindChildrenByName<CodeClass>(x.Name)
-                .Except([parentClass]).OfType<CodeClass>() // the property method should not reference itself as a return type. 
+                .Except(exceptions)// the property method should not reference itself as a return type. 
                 .MinBy(shortestNamespaceOrder);
             // searching down first because most request builder properties on a request builder are just sub paths on the API
             if (x.TypeDefinition == null)