setState(…): Can only update a mounted or mounting component. This usually means you called setState() on an unmounted component. This is a no-op

I solved this by assigning a ref to the component and then checking if the ref exists before setting the state:

myMethod(){
  if (this.refs.myRef) 
   this.setState({myVar: true});
}

render() {
  return (
    <div ref="myRef">
      {this.state.myVar}
    </div>
  );
}

And lately, since I am using mostly functional components, I am using this pattern:

const Component = () => {
  const ref = React.useRef(null);
  const [count, setCount] = React.useState(0);

  const increment = () => {
    setTimeout(() => { // usually fetching API data here
      if (ref.current !== null) {
        setCount((count) => count + 1);
      }
    }, 100);
  };

  return (
    <button onClick={increment} ref={ref}>
      Async Increment {count}
    </button>
  );
};