This is why flow invalidates type refinements at seemingly arbitrary times

refinement.js

class Test {
  constructor() {
    this.scrollView = {
      flash: () => {
        console.log(this);
      },
      doSomethingElse: () => {
        console.log('*evil laugh*');
        this.scrollView = null;
      },
    }
  }
}

const t = new Test();
if (t.scrollView) {
  t.scrollView.doSomethingElse();
  // => *evil laugh*
  t.scrollView.flash();  // with flow enabled, this line will error, because the refinement is nullfied by the previous function call
  // => Uncaught TypeError: Cannot read property 'flash' of null
}