class='MJX-TeXAtom-ORD' in MathML crashes mathml2chtml

Created by: NSoiffer

Issue Summary

If class='MJX-TeXAtom-ORD' is present in MathML being converted by mathml2chtml, then the function crashes. This happens in both Firefox and Chrome

Steps to Reproduce:

1. Open https://mathjax.github.io/MathJax-demos-web/input-mml2chtml.html
2. Paste the following MathML into the input area:

 <math display='block' id='Mna33kyx-0'>
    <mn class='MJX-TeXAtom-ORD' id='Mna33kyx-9'>2</mn>
 </math>

3. Click Render and open the debugger to see the message:

Uncaught Error: Unexpected text node "2"
    at t.error (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:205225)
    at t.addText (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:204033)
    at t.addChildren (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:203371)
    at t.makeNode (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:202222)
    at t.addChildren (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:203503)
    at t.makeNode (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:202222)
    at t.addChildren (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:203503)
    at t.makeNode (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:202222)
    at t.addChildren (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:203503)
    at t.makeNode (cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js:1:202222)

I know it might seem strange to have this internal class set, but the situation is that the math is being grabbed from the web, processed a little, and being sent back to MathJax for rendering. Since the original source (Wikipedia) used MathJax, that shows up.

Technical details:

• MathJax V3.2.0