Update DerivedTypesTreeNode with == null instead of != null.

it appears Fix #3263 added an accidental `!=` instead of `==` and broke the derived types node.

ILSpy/TreeNodes/DerivedTypesTreeNode.cs

@@ -73,7 +73,7 @@ namespace ICSharpCode.ILSpy.TreeNodes
 					continue;
 				var metadata = module.Metadata;
 				var assembly = module.GetTypeSystemOrNull()?.MainModule as MetadataModule;
-				if (assembly != null)
+				if (assembly == null)
 					continue;
 				foreach (var h in metadata.TypeDefinitions)
 				{
@@ -107,4 +107,4 @@ namespace ICSharpCode.ILSpy.TreeNodes
 			threading.Decompile(language, output, options, EnsureLazyChildren);
 		}
 	}
-}
+}