Always assign providersByName to avoid NullReferenceException

Co-authored-by: brianrob <6210322+brianrob@users.noreply.github.com>

Author: Copilot

src/TraceEvent/TraceEventSession.cs

@@ -1796,9 +1796,10 @@ internal static SortedDictionary<string, Guid> ProviderNameToGuid
                                     var name = new string((char*)&buffer[providers[i].ProviderNameOffset]);
                                     providersByName[name] = providers[i].ProviderGuid;
                                 }
-
-                                s_providersByName = providersByName;
                             }
+
+                            // Always assign providersByName to avoid NullReferenceException on subsequent lookups
+                            s_providersByName = providersByName;
                         }
                     }
                 }