chore: apply copilot suggestion

Author: MaggieKimani1

src/Microsoft.OpenApi/Reader/ParsingContext.cs

@@ -271,10 +271,10 @@ public void PopLoop(string loopid)
 
         private void ValidateRequiredFields(OpenApiDocument doc, string version)
         {
-            if ((version.is2_0() || version.is3_0()) && (doc.Paths == null))
+            if ((version.is2_0() || version.is3_0()) && (doc.Paths == null) && RootNode is not null)
             {
                 // paths is a required field in OpenAPI 2.0 and 3.0 but optional in 3.1
-                RootNode?.Context.Diagnostic.Errors.Add(new OpenApiError("", $"Paths is a REQUIRED field at {RootNode.Context.GetLocation()}"));
+                RootNode.Context.Diagnostic.Errors.Add(new OpenApiError("", $"Paths is a REQUIRED field at {RootNode.Context.GetLocation()}"));
             }
         }
     }