[app-state] more informative error message when assigning to an undefined state

[RaphiStein]

When assigning to a state that is undefined, (i.e. store('mystate').state() === undefined), the library produces an error that is confusing and does not actually tell me what I'm doing wrong.

i.e.

store('mystate').assign(undefined)
store('mystate').assign({age: 30, name: 'joe'})

produces a console error saying:

preview-bc361a02a95aae7c3363b.js:2 TypeError: Cannot read property 'age' of undefined
    at eval (store.ts:73)
    at eval (every.ts:27)

Desired behaviour

A message saying "cannot assign to undefined property mystate"
(and perhaps include the application code line number in the stack trace?)

Stackblitz link:
https://stackblitz.com/edit/typescript-qg4jq7

[ersimont]

I like your profile pic :)

Thanks for the suggestion!

[ersimont]

This will come with version 13