Filter is overzealous in Adapt.findRelativeModel

[tomgreenfield]

Subject of the issue/enhancement/features

options.filter in Adapt.findRelativeModel erroneously filters out the current model producing inconsistencies:

adapt_framework/src/core/js/models/adaptModel.js

Lines 499 to 502 in 5a3de69

	// filter if opinionated
	if (typeof options.filter === 'function') {
	pageDescendants = pageDescendants.filter(options.filter);
	}

Your environment

Framework 5.10

Steps to reproduce

In the core-bundled course, run the following in the console to make co-10 optional and attempt to retrieve its previous (mandatory) page ID:

require('core/js/adapt').findById('co-10')
  .set('_isOptional', true)
  .findRelativeModel('contentobject -1', {
    filter: model => !model.get('_isOptional')
  }).get('_id');

Expected behaviour

"co-05" should be returned from the above code.

Actual behaviour

The following is returned because co-10 filters itself out:

adaptModel.js:527 Uncaught TypeError: Cannot read property 'isTypeGroup' of undefined
    at PageModel.findRelativeModel (adaptModel.js:527)
    at <anonymous>:3:4