Unknown node type "body" when compiling MathML in v3.0.1

Created by: hubgit

Using mathjax-full@3.0.0 it was possible to use this code to convert MathML to SVG:

import { MathML } from 'mathjax-full/js/input/mathml'
import { SVG } from 'mathjax-full/js/output/svg'
import { HTMLAdaptor } from 'mathjax-full/js/adaptors/HTMLAdaptor'
import { HTMLDocument } from 'mathjax-full/js/handlers/html/HTMLDocument'

const doc = new HTMLDocument(document, new HTMLAdaptor(window), {
  InputJax: new MathML(),
  OutputJax: new SVG(),
})

const svg = doc.convert(`<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>A</mi><mo>=</mo></mrow></math>`)

console.log(svg.innerHTML)

Using mathjax-full@3.0.1 an error is thrown when using the same code:

Uncaught Error: Unknown node type "body"
    at MathMLCompile.error (MathMLCompile.ts:320)
    at MathMLCompile.makeNode (MathMLCompile.ts:125)
    at MathMLCompile.compile (MathMLCompile.ts:95)
    at MathML.compile (mathml.ts:145)
    at HTMLMathItem.AbstractMathItem.compile (MathItem.ts:327)
    at Object.renderMath (MathDocument.ts:167)
    at RenderList.renderConvert (MathDocument.ts:211)
    at HTMLMathItem.AbstractMathItem.convert (MathItem.ts:319)
    at HTMLDocument.AbstractMathDocument.convert (MathDocument.ts:633)

It seems to go wrong around this line in MathDocument:

mitem.start.node = this.adaptor.body(this.document)

which sets mitem.start.node to the body of this.document, which suggests that this commit might be to blame.