Uncaught Error: reentry detected, value=null, oldValue=9

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Noted by @KatieWoe during ES6 module migration testing. Unknown whether this is related to ES6 module migration.

Steps to reproduced:

1. Start the sim with ?ea
2. Go to Shopping Lab screen
3. Enter '9' in the Double Number Line numerator
4. Enter '9' in the Double Number Line denominator

This Error occurs:

Uncaught Error: Assertion failed: reentry detected, value=null, oldValue=9
    at window.assertions.assertFunction (assert.js?bust=1583857607570:22)
    at Property._notifyListeners (Property.js:273)
    at Property.set (Property.js:175)
    at Property.reset (Property.js:339)
    at MarkerEditor.reset (MarkerEditor.js:68)
    at markerEditorObserver (DoubleNumberLineAccordionBox.js:164)
    at TinyEmitter.emit (TinyEmitter.js:65)
    at Property._notifyListeners (Property.js:276)
    at Property.set (Property.js:175)
    at Property.set value [as value] (Property.js:346)

The numerator value gets cleared and the sim looks like this:

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Using the shas that were recorded just before ES6 module migration (see https://github.com/phetsims/chipper/issues/875#issuecomment-592105084), I demonstrated that this problem occurred before migration, and is therefore unrelated to migration.

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Also fails on the Shopping screen with denominator 5 and numerator 2, which happens to be a "correct" answer.

Uncaught Error: Assertion failed: reentry detected, value=null, oldValue=2
    at window.assertions.assertFunction (assert.js?bust=1583290850331:22)
    at Property._notifyListeners (Property.js:273)
    at Property.set (Property.js:175)
    at Property.reset (Property.js:339)
    at MarkerEditor.reset (MarkerEditor.js:68)
    at markerEditorObserver (DoubleNumberLineAccordionBox.js:164)
    at TinyEmitter.emit (TinyEmitter.js:65)
    at Property._notifyListeners (Property.js:276)
    at Property.set (Property.js:175)
    at Property.set value [as value] (Property.js:346)

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Fixed, closing.