Unexpected token u in JSON at position 0

[FractalMind]

ng-walkthrough behave very weirdly in my app.

1- When I load the page the "arrow" works but the CSS inside ng-walkthrough doesn't register and I get this error:

core.js:4002 ERROR TypeError: Cannot read property 'top' of null
    at WalkthroughContainerComponent.push../node_modules/angular-walkthrough/fesm5/angular-walkthrough.js.WalkthroughContainerComponent.contentBlockPosition (angular-walkthrough.js:1767)
    at angular-walkthrough.js:1203
    at ZoneDelegate.invokeTask (zone.js:431)
    at Object.onInvokeTask (core.js:26247)
    at ZoneDelegate.invokeTask (zone.js:430)
    at Zone.runTask (zone.js:198)
    at ZoneTask.invokeTask (zone.js:513)
    at ZoneTask.invoke (zone.js:502)
    at timer (zone.js:3041)
defaultErrorLogger @ core.js:4002
push../node_modules/@angular/core/fesm5/core.js.ErrorHandler.handleError @ core.js:4050
next @ core.js:26759
schedulerFn @ core.js:23735
push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.__tryOrUnsub @ Subscriber.js:192
push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.next @ Subscriber.js:130
push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber._next @ Subscriber.js:76
push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next @ Subscriber.js:53
push../node_modules/rxjs/_esm5/internal/Subject.js.Subject.next @ Subject.js:47
push../node_modules/@angular/core/fesm5/core.js.EventEmitter.emit @ core.js:23719

2- if I navigate to another page and come back now the arrow disappear, the CSS register and I get this wierd error:

ERROR SyntaxError: Unexpected token u in JSON at position 0
    at JSON.parse (<anonymous>)
    at WalkthroughContainerComponent.push../node_modules/angular-walkthrough/fesm5/angular-walkthrough.js.WalkthroughContainerComponent.contentBlockPosition (angular-walkthrough.js:1766)
    at angular-walkthrough.js:1203
    at ZoneDelegate.invokeTask (zone.js:431)
    at Object.onInvokeTask (core.js:26247)
    at ZoneDelegate.invokeTask (zone.js:430)
    at Zone.runTask (zone.js:198)
    at ZoneTask.invokeTask (zone.js:513)
    at ZoneTask.invoke (zone.js:502)
    at timer (zone.js:3041)

Do you have an idea on how to resolve this issue?

Thank you for your time

"@angular/common": "8.2.14", "@ionic-native/core": "5.18.0", "angular-walkthrough": "^0.8.2",