Cannot read property ‘sendWithPromise’ of null error

Bug Report or Feature Request (mark with an x)

- [ ] Regression (a behavior that used to work and stopped working in a new release)
- [X] Bug report -> please search issues before submitting
- [ ] Feature request
- [ ] Documentation issue or request

I’ve got an error log from one of customers.

TypeError: Cannot read property 'sendWithPromise' of null
  at e.getPage(/main.fcbd366b59adeaccde73.js:1:2580545)
  at e.getPage(/main.fcbd366b59adeaccde73.js:1:2562311)
  at e.renderPage(/main.fcbd366b59adeaccde73.js:1:2100087)
  at e.updateSize(/main.fcbd366b59adeaccde73.js:1:2098625)
  at ? (/main.fcbd366b59adeaccde73.js:1:2095964)
  at i(/main.fcbd366b59adeaccde73.js:1:2155073)
  at t.invokeTask(/polyfills.6daafc40a5cb42269e22.js:1:8745)
  at Object.onInvokeTask(/main.fcbd366b59adeaccde73.js:1:313689)
  at t.invokeTask(/polyfills.6daafc40a5cb42269e22.js:1:8666)
  at e.runTask(/polyfills.6daafc40a5cb42269e22.js:1:3929)
  at e.invokeTask(/polyfills.6daafc40a5cb42269e22.js:1:9835)
  at invoke(/polyfills.6daafc40a5cb42269e22.js:1:9726)
  at n.args.(anonymous function)(/polyfills.6daafc40a5cb42269e22.js:1:25027)

I couldn’t reproduce the issue but here are my thoughts.

It seems that this error appears after a user tries to change page index in the PDF preview.
The piece of code where the issue happenes looks like this

var l=this.messageHandler.sendWithPromise("GetPage",{pageIndex:t}).then(function(e){if(n.destroyed)throw new Error("Transport destroyed");var l=new M(t,e,n,n._params.pdfBug);

Since the issue happens after renderPage and getPage my guess is that the source of issue is the not initialized _pdf variable which is at 388 line of this file
https://github.com/VadimDez/ng2-pdf-viewer/blob/master/src/app/pdf-viewer/pdf-viewer.component.ts

Could it be that I’m right and that additional check like this

if (!this._pdf) {
      return;
    }

could fix the issue?
Thanks