Create a new xml file in the project, the root layout file appears red, and prompts: The layout "xxx" in layout has no declaration in the base layout folderMissingDefaultResource, etc. (It will prompt missing or something, let you add this class), and it cannot be loaded when loading this layout. The solution at this time is as follows:
1. Open File in AS
2. Click Invalidate Caches
3. Click Invalidate and Restart