How to reproduce:
1. Get project from from Collab ([Mobile] Touch Input & Keyboards) or download provided one
2. Notice it has a scripting error "The type or namespace name 'Canvas' could not be found (are you missing a using directive or an assembly reference?)". It requires UI package
3. Check Package folder and notice that it is empty
4. Open Window -> Package Manager to get one
Result: Package manager window is blank and it is not possible to get UI package and use the project

Reproducible: 2018.2.0b8, 2018.2.2f1, 2018.3.0a7