Accessing computed properties in combination with extendObservable causes crash

[deadbeef84]

I have an issue:

• Provide error messages including stacktrace

TypeError: Cannot read property 'get' of undefined
    at ObservableObjectAdministration$$1.read (VM455 mobx.js:3477)
    at AppState.get (VM455 mobx.js:3737)

• Provide a minimal sample reproduction.

https://codesandbox.io/s/1ynlm8v7k7

• Did you check this issue wasn't filed before?

Couldn't find any similar issue.

• Elaborate on your issue. What behavior did you expect?

Accessing computed properties fails on the second instance. In this minimal reproduction it appears you can access the property after instantiation; however this is not the case in my larger code base (could it be dependent on from where it is instantiated? I'm thinking it's in tracked context in my larger code base).

Workarounds:

• Using @observable fixes the issue.

• Accessing the property before extendObservable(...) makes the error go away

  • State the versions of MobX and relevant libraries. Which browser / node / ... version?

• mobx 5.1.2

• chrome 69.0.3497.100

• node 10.9.0

[mweststrate]

Thanks for the accurate test! Fixed in MobX 4.5.0 / 5.5.0