Karma/Jasmine Test 'Cannot read property 'length' of null' Error

For unknown reasons, only some of my karma tests started to fail with the following error:

TypeError: Cannot read property 'length' of null

NavControllerBase.prototype.destroy = function () {
        var /** @type {?} */ views = this._views;
        var /** @type {?} */ view;
        for (var /** @type {?} */ i = 0; i < views.length; i++) {
            view = views[i];
            view._willUnload();
            view._destroy(this._renderer);
        }
        ...
    };

It seems as though we are not able to destroys for some reason.

Any insights on this would be much appreciated.