Thanks for reaching out to PhET and sorry for the trouble. That error is from our linting software (code quality enforcer). At PhET we have a need to seed a sim to have identical runtimes. Because of this, we use a single instance of Random called `DOT/dotRandom`. You can take a look at usages of that and see if it would work for you.
Otherwise, this is just a lint error, and not an actual problem with the code. To get around it you can do any of the following:
* add `--lint=false` to the build command, like `grunt --brands=adapted-from-phet --lint=false`
* add `// eslint-disable-line bad-sim-text` to the end of the line that you are creating or using Random on. If you search for this string in our codebase, you should see other usages just like yours where we are alright with an unseeded random instance (like https://github.com/phetsims/kite/blob/bc7e09afea97bab68aad30aeb0fd9e57bd4fd515/js/Shape.js#L38
I hope this is helpful. Please let us know if you run into any more issues.
PhET Interactive Simulations
University of Colorado