I have some web parts that are dependent on a couple of custom SharePoint lists. If the lists didn't exist or didn't have the fields the web parts needed when the web parts were added to a page, the whole page would bomb out. After I came up with my solution, I wrote an article on it and thought I'd share it with you. This how-to also shows how the web parts can be added to a page with a message telling the user to configure the list source property, and once a user does, the web part renders as normal. I also show how to verify the list exists and has the fields necessary for the web part to function. Its all in this article, How to set a SharePoint web part's list at run time