The exception will no longer throw if you use version Telerik UI for WinForms R1 2017 (2017.1.117). However, calling the PerformClick method on the NextNavigationButton or the PrevNavigationButton won`t collapse or expand the split panel. This is a known issue which is already logged in our Feedback Portal. You can track its progress, subscribe for status changes and add your comment to it on the case page: https://feedback.telerik.com/Project/154/Feedback/Details/211163-add-radsplitcontainer-expose-api-for-accessing-the-splitter-buttons-and-perfor