Can you share more code on how you are getting the splitview instance? It's likely that the view hierarchy to the main splitview has changed and you will need to update how that's being found.
Happy to help figure out the issue! However, if you aren't sure how to fix it yourself its probably best to stick to writing plugins that don't inject UI into Sketch. It's much safer and you don't have to juggle trying to support old versions of Sketch that have different view hierarchies. I highly recommend a floating panel positioned in a helpful spot rather than trying to upkeep your plugin each time Sketch changes things.