my_class.js

// @flow

declare function SomeFn(): void;

type A = {
    type: 'simple';
    foo: number;
    requiredName: string;
};

type B = {
    type: 'error';
    bar: number;
    errorHandler: SomeFn;
};

type AorB = A | B;

class MyClass {
    _states: {[key: string]: AorB};
    constructor() {
        this._states = {};
    }
    addB(n: string): void {
        this._states[n] = {
            type: 'error',
            foo: 1,
        };
    }
}

fwiw this catches the mistake, not sure what's going on in your example :/

type A = {
    type: 'simple';
    foo: number;
    requiredName: string;
};

type B = {
    type: 'error';
    bar: number;
};

type AorB = A | B;

let foo: A = {type: 'simple', foo: 2, requiredName: 'asdf'};
let bar: B = {type: 'error', bar: 3};

let obj: {[key: string]: AorB} = {};
obj['a'] = foo;
obj['b'] = bar;
obj['c'] = {type: 'error', foo: 1}; // This type is incompatible with union: A | B