[CommonHTML] regression on develop branch: horizontal stretch causes processing error

I ran into a regression on the develop branch.

Here's a minimal example. It seems to require the mtable and the mover (this came from an AMScd source).

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mtable rowspacing="5pt" columnspacing="5pt" displaystyle="true">
    <mtr>
      <mtd>
        <mi>F</mi>
      </mtd>
      <mtd>
        <mover>
          <mo minsize="2.75em">&#x2192;</mo>
        </mover>
      </mtd>
      <mtd> 
        <mi>F</mi>
      </mtd>
    </mtr>
  </mtable>
</math>

lastError gave me:

TypeError: Cannot read property 'firstChild' of null
    at CHTMLstretchH (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1959:22)
    at CHTMLstretchCoreH (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1472:28)
    at CHTMLstretchH (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1704:14)
    at CHTMLstretchCoreH (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1472:28)
    at CHTMLstretchH (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1468:36)
    at CHTMLstretchCells (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/autoload/mtable.js?rev=2.6.1:422:55)
    at toCommonHTML (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/autoload/mtable.js?rev=2.6.1:71:12)
    at CHTMLaddChild (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1395:25)
    at CHTMLdefaultNode (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1380:42)
    at toCommonHTML (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:2604:21)
    at CHTMLaddChild (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1395:25)
    at CHTMLdefaultNode (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1380:42)
    at toCommonHTML (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:1733:21)
    at Translate (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:507:14)
    at Object.processOutput (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:2306:51)
    at Function.execute (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:236:26)
    at Function.WAITEXECUTE (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:337:31)
    at cb (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:218:59)
    at Function.WAITEXECUTE (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:341:50)
    at cb (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:218:59)
    at Execute (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:419:36)
    at Object.loadComplete (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:951:35)
    at Function.execute (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:236:26)
    at cb (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:218:59)
    at Process (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:488:38)
    at Push (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:478:49)
    at Init (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:463:17)
    at Function.Init (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:88:23)
    at Function.Queue (https://rawgit.com/mathjax/MathJax/develop/unpacked/MathJax.js:66:54)
    at fontLoaded (https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/jax.js?rev=2.6.1:334:24)
    at https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/fonts/TeX/AMS-Regular.js?rev=2.6.1:287:7
    at https://rawgit.com/mathjax/MathJax/develop/unpacked/jax/output/CommonHTML/fonts/TeX/AMS-Regular.js?rev=2.6.1:289:3