We recreated this issue when we upgrade to the latest Xamarin and Xcode (It works fine with our code projects, I think it should be some difference on the plist file). Not sure whether Xamarin or Apple changed something. We will contact them to see if there is any tricky.
On the other hand, we played various settings with this project, and found if we set the linker to "Link All Assemblies", it works.
Another issue in your code is that, the parameter you passed in ShapeFileFeatureLayer.BuildIndexFile method is a private path. This folder is not writable. In this case, it is failed building index here and the code below won't process. I have built the index and include in the attached sample. Please try this one and let you know when we have an update from Xamarin Supports.
004_003_002_001_HelloWorld.zip (1.59 MB)