macOS Mojave: invalid active developer path after updating to Mojave from High Sierra

Open Terminal, and run the following:

xcode-select --install

This will download and install xcode developer tools and fix the problem. The problem is that one needs to explicitly agree to the license agreement. As a follow on step, you may need to reset the path to Xcode if you have several versions or want the command line tools to run without Xcode.

xcode-select --switch /Applications/Xcode.app

xcode-select --switch /Library/Developer/CommandLineTools

I have also found the solution in this question.

https://apple.stackexchange.com/questions/254380/macos-mojave-invalid-active-developer-path