Credits to kharek for his answer here. But his answer was for an older version (2.8). There are some minor tweaks for getting it to work on the latest version (StarUML-3.0.2-x86_64.AppImage).

Here's a complete guide (for newbies) (it worked for me on Ubuntu 18.04.1 LTS):

1. Download the latest StarUML .AppImage from the their website staruml.io
2. Then make the downloaded .AppImage executable by running sudo chmod +x StarUML-3.0.2-x86_64.AppImage
3. Install npm using apt-get sudo apt install npm
4. Install asar npm package using sudo npm install -g asar
5. If you're using npm for the first time, then you can't directly call asar from the terminal. You need to update your $PATH variable to include the .npm-global directory to directly call globally installed npm packages. This can be done by adding export PATH="/home/$USER/.npm-global/bin:$PATH" (may differ based on your config; refer to this SO answer) at the end of your .bashrc. Then logout and login again.
6. Then extract .AppImage file using ./StarUML-3.0.2-x86_64.AppImage --appimage-extract
7. cd squashfs-root/resources/
8. asar extract app.asar app
9. cd app/src/engine
10. Edit the license-manager.js file. Make the following changes:

//... existing code
checkLicenseValidity () {
    this.validate().then(() => {
      setStatus(this, true)
    }, () => {
      //setStatus(this,false)  <-- comment this line
      setStatus(this, true) //<-- add this line
      //UnregisteredDialog.showDialog() <-- comment this line
    })
  }
//... rest of the code

11. cd ../../../
12. Pack the updated .AppImage using asar pack app app.asar
13. Remove the previously extracted app directory by running rm -rf app/* followed by rmdir app
14. cd ../../
15. Download the appimagetool from here to your current directory
16. Run this ./appimagetool-x86_64.AppImage squashfs-root/ to generate the new .AppImage file
17. Run the new .AppImage file by running ./StarUML-x86_64.AppImage