weird `Method cannot be called on possibly null / undefined value` - flowtype

Question

The following narrowed-down code:
// #flow
'use strict';
import assert from 'assert';
class Node<V, E> {
value: V;
children: ?Map<E, Node<V,E>>;
constructor(value: V) {
this.value = value;
this.children = null;
}
}
function accessChildren(tree: Node<number, string>): void {
if (tree.children!=null) {
assert(true); // if you comment this line Flow is ok
tree.children.forEach( (v,k)=>{});
} else {
}
}
… fails Flow type checking with the following message:
$ npm run flow
> simple-babel-serverside-node-only-archetype#1.0.0 flow /home/blah/blah/blah
> flow; test $? -eq 0 -o $? -eq 2
es6/foo.js:21
21: tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call of method `forEach`. Method cannot be called on possibly null value
21: tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^ null
es6/foo.js:21
21: tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call of method `forEach`. Method cannot be called on possibly undefined value
21: tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^ undefined
Found 2 errors
If the line reading: assert(true) is commented out, flow is satisfied !
What gives?
PS: In case anyone wonders, my .flowconfig, .babelrc and package.json files are nondescript:
.flowconfig
$ cat .flowconfig
[options]
esproposal.class_static_fields=enable
.babelrc
$ cat .babelrc
{
"presets": ["es2015"],
"plugins": ["transform-object-rest-spread", "transform-flow-strip-types", "transform-class-properties"]
}
package.json
$ cat package.json
{
"name": "simple-babel-serverside-node-only-archetype",
"version": "1.0.0",
"description": "",
"main": [
"index.js"
],
"scripts": {
"build": "babel es6 --out-dir es5 --source-maps",
"build-watch": "babel es6 --out-dir es5 --source-maps --watch",
"start": "node es5/index.js",
"flow": "flow; test $? -eq 0 -o $? -eq 2"
},
"author": "",
"license": "ISC",
"devDependencies": {
"babel-cli": "^6.6.5",
"babel-core": "^6.7.4",
"babel-plugin-transform-class-properties": "^6.10.2",
"babel-plugin-transform-flow-strip-types": "^6.8.0",
"babel-polyfill": "^6.7.4",
"babel-preset-es2015": "^6.9.0",
"babel-runtime": "^6.6.1",
"flow-bin": "^0.27.0"
},
"dependencies": {
"babel-plugin-transform-object-rest-spread": "^6.8.0",
"babel-polyfill": "^6.7.4",
"source-map-support": "^0.4.0"
}
}

Answer 1

Your case is described here.
Flow cannot know, that assert doesn't change the tree.
Add the following lines to your code and run it – you will have runtime error, because assert function will set tree.children to null when called.
const root = new Node(1);
const child = new Node(2);
root.children = new Map([['child', child]]);
assert = () => root.children = null;
accessChildren(root);
Yes, it is pretty weird code, but Flow doesn't know, you will not to write it.

Answer 2

Others have pointed to the right explanation. Fortunately this works:
// #flow
'use strict';
import assert from 'assert';
class Node<V, E> {
value: V;
children: ?Map<E, Node<V, E>>;
constructor(value: V) {
this.value = value;
this.children = null;
}
}
function accessChildren(tree: Node<number, string>): void {
const children = tree.children; // save possibly mutable reference to local
if (children != null) {
assert(true); // if you comment this line Flow is ok
children.forEach((v, k) => {});
} else {
}
}
Also, in the future Flow will have read-only properties and by declaring children as a read-only property in the class, Flow should be able to preserve type check the original code.