What you can also do is manually download Xcode and install it.
(Without using the app store update)
Therefor do the following steps:
- go to https://developer.apple.com/download/more/
- search for Xcode (or latest version e.g Xcode 10.2) and manually download and install it.
See also this post here: https://discussions.apple.com/thread/8622103?answerId=250008933022#250008933022