If a upm package is added using a directory outside of the project root, and then you attempt to create a U# script inside of that external package, an exception is thrown and the asset file is not created. This is because UdonSharpBehaviourEditor fails to convert the absolute path to an AssetDatabase path in this situation.

Instead of using EditorUtility.SaveFilePanel and then trying to make it into a relative path, it should be using EditorUtility.SaveFilePanelInProject.