Destroy hook of agm-direction directive fails on back browser button click

[vleput]

Issue description

My app crashes and getting a weird error message in the console from the agm-direction library when hitting the "back" button of the browser (only tested in Chrome).
My map is in a lazy-loaded module, and all agm-* related components are in a single component that is in that lazy-loaded module.
Below is the message in the console:

TypeError: Cannot read property 'setPanel' of undefined
    at AgmDirection.push../node_modules/agm-direction/fesm5/agm-direction.js.AgmDirection.removeDirections (agm-direction.js:306)
    at AgmDirection.push../node_modules/agm-direction/fesm5/agm-direction.js.AgmDirection.ngOnDestroy (agm-direction.js:100)
    at callProviderLifecycles (core.js:18860)

It seems like directionsDisplay is undefined when running the ngOnDestroy hook of the agm-direction directive?

Steps to reproduce the issue

1. Navigate to Angular app, home route
2. Click on a /map route that loads a lazy-loaded module containing the agm-direction directive
3. Click on the back button of the browser
4. App crashes: component disappears from DOM, route stays the same, and the error message above is logged in the console

What's the expected result?

• agm-direction directive destroys itself gracefully

What's the actual result?

• There seems to be a bug in the ngOnDestroy hook of the agm-direction component

[vleput]

The map part of the app I inherited is pretty buggy so might be why I'm getting this. Still might be nice to check whether directionsDisplay https://github.com/explooosion/Agm-Direction/blob/master/src/directive/agm-direction.directive.ts#L54 is not undefined on the destroy hook