[Modal Ref] Accessing componentInstance after a modal was closed raises a javascript error

[loicgasser]

Bug description:

Accessing componentInstance after a modal was closed raises a javascript error.

Link to minimally-working StackBlitz that reproduces the issue:

demo

Steps to reproduce in the sample.

1. Click Launch demo modal
2. Close the modal
3. Click Raise componentInstance error

Result:

TypeError: Cannot read property 'componentRef' of null

Expected Result:

componentInstance is undefined

The demo demonstrates a simplified version of the problem.
I think this could be useful when you have a NgbModalRef in something like ngOnChanges and you are trying to determine if the modal is open to change its Inputs.

It would allow you to do things like

    if (this._modalRef && this._modalRef.componentInstance) {
      console.log('do something');
    }

which overall would provide more flexibility.

Versions of Angular, ng-bootstrap and Bootstrap:

Angular: 8.0.0

ng-bootstrap: 5.1.1

Bootstrap: 4.3.1