Deal with disconnected elems in mathml.isMathJaxIgnored

I didn't take into account we might get handed an element that's not
attached to the body.

closes LS-1715
flag=none

test plan:
  - load /courses/:id/pages
  > expect no exception to be thrown from mathml.isMathJaxIgnored
    in particular:
    Uncaught TypeError: Cannot read property 'querySelector' of null
    or
    Uncaught TypeError: Cannot read property 'classList' of nul

Change-Id: I079b57521574b5ce7a3ed11ced4a985d47762e04
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/255380
Reviewed-by: Jeff Largent <jeff.largent@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Robin Kuss <rkuss@instructure.com>
Product-Review: Ed Schiebel <eschiebel@instructure.com>

public/javascripts/mathml.js

@@ -143,12 +143,17 @@ const mathml = {
   },
 
   isMathJaxIgnored(elem) {
+    if (!elem) return true
+
+    // ignore disconnected elements
+    if (!document.body.contains(elem)) return true
+
     // elements to ignore selector
     const ignore_list =
       '.MJX_Assistive_MathML,#header,#mobile-header,#left-side,#quiz-elapsed-time,.ui-menu-carat'
 
     // check if elem is in the ignore list
-    if (elem.parentElement.querySelector(ignore_list) === elem) {
+    if (elem.parentElement?.querySelector(ignore_list) === elem) {
       return true
     }
 

spec/javascripts/jsx/mathmlSpec.js

@@ -314,3 +314,13 @@ test('ignores descendents of .mathjax_ignore', () => {
   ignored.appendChild(elem)
   ok(mathml.isMathJaxIgnored(elem))
 })
+
+test('deals with disconnected elements', () => {
+  // even though they should never get here
+  const elem = document.createElement('span')
+  ok(mathml.isMathJaxIgnored(elem))
+})
+
+test('handles missing element', () => {
+  ok(mathml.isMathJaxIgnored())
+})