Items disappeared from model, then model and some items became unaccessible

Setup, Configuration and Use Items & Sitemaps
5

It now occurred to me to look at the browser console for errors when trying to load the semantic model (at ../settings/model/) and this is what I found:

content.js:514 Uncaught TypeError: Te.$hook is not a function
    at new <anonymous> (content.js:514)
    at content.js:513
    at XMLHttpRequest.V.onreadystatechange (content.js:511)
app.js:7 TypeError: Cannot read property 'val' of undefined
    at s.value (app.js:7)
    at i.<anonymous> (16.app.js:1)
    at Array.<anonymous> (app.js:7)
    at Ze (app.js:7)
Ge @ app.js:7
app.js:7 TypeError: Cannot read property 'val' of undefined
    at s.value (app.js:7)
    at i.<anonymous> (16.app.js:1)
    at Array.<anonymous> (app.js:7)
    at Ze (app.js:7)
Ge @ app.js:7
app.js:7 TypeError: Cannot read property 'val' of undefined
    at s.value (app.js:7)
    at i.<anonymous> (16.app.js:1)
    at Array.<anonymous> (app.js:7)
    at Ze (app.js:7)
Ge @ app.js:7
app.js:7 TypeError: Cannot read property 'val' of undefined
    at s.value (app.js:7)
    at i.<anonymous> (16.app.js:1)
    at Array.<anonymous> (app.js:7)
    at Ze (app.js:7)
Ge @ app.js:7
app.js:7 TypeError: Cannot read property 'val' of undefined
    at s.value (app.js:7)
    at i.<anonymous> (16.app.js:1)
    at Array.<anonymous> (app.js:7)
    at Ze (app.js:7)
Ge @ app.js:7
app.js:7 TypeError: Cannot read property 'val' of undefined
    at s.value (app.js:7)
    at i.<anonymous> (16.app.js:1)
    at Array.<anonymous> (app.js:7)
    at Ze (app.js:7)
Ge @ app.js:7
16.app.js:1 Uncaught TypeError: Cannot read property 'update' of undefined
    at 16.app.js:1
app.js:7 Uncaught (in promise) RangeError: Maximum call stack size exceeded
    at new Ee (app.js:7)
    at Te (app.js:7)
    at $e (app.js:7)
    at Ee.walk (app.js:7)
    at new Ee (app.js:7)
    at Te (app.js:7)
    at $e (app.js:7)
    at Ee.walk (app.js:7)
    at new Ee (app.js:7)
    at Te (app.js:7)
app.js:7 Uncaught (in promise) RangeError: Maximum call stack size exceeded
    at new Ee (app.js:7)
    at Te (app.js:7)
    at $e (app.js:7)
    at Ee.walk (app.js:7)
    at new Ee (app.js:7)
    at Te (app.js:7)
    at $e (app.js:7)
    at Ee.walk (app.js:7)
    at new Ee (app.js:7)
    at Te (app.js:7)
DevTools failed to load source map: Could not load content for chrome-extension://hponfflfgcjikmehlcdcnpapicnljkkc/static/js/smartquotes.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
app.js:7 Uncaught (in promise) RangeError: Maximum call stack size exceeded
    at Ee.walk (app.js:7)
    at new Ee (app.js:7)
    at Te (app.js:7)
    at Ee.observeArray (app.js:7)
    at new Ee (app.js:7)
    at Te (app.js:7)
    at Object.set [as equipment] (app.js:7)
    at i.getChildren (16.app.js:1)
    at Array.forEach (<anonymous>)
    at i.getChildren (16.app.js:1)
Promise.then (async)
load @ 16.app.js:1
onPageAfterIn @ 16.app.js:1
qe @ app.js:7
n @ app.js:7
qe @ app.js:7
e.$emit @ app.js:7
(anonymous) @ app.js:7
t.a @ app.js:7
dispatchEvent @ app.js:23
onPageAfterIn @ app.js:23
(anonymous) @ app.js:7
(anonymous) @ app.js:7
value @ app.js:7
(anonymous) @ app.js:7
value @ app.js:7
(anonymous) @ app.js:7
value @ app.js:7
value @ app.js:7
W @ app.js:7
D.forward @ app.js:7
v @ app.js:7
e @ app.js:23
(anonymous) @ app.js:7
(anonymous) @ app.js:7
value @ app.js:7
updated @ app.js:23
qe @ app.js:7
tn @ app.js:7
(anonymous) @ app.js:7
pn @ app.js:7
(anonymous) @ app.js:7
Ze @ app.js:7
Promise.then (async)
Ye @ app.js:7
at @ app.js:7
(anonymous) @ app.js:7
fn.update @ app.js:7
pe.notify @ app.js:7
(anonymous) @ app.js:7
pageComponentLoader @ app.js:23
D.load @ app.js:7
t @ app.js:7
(anonymous) @ app.js:33
Promise.then (async)
(anonymous) @ app.js:33
b @ app.js:7
(anonymous) @ app.js:7
o @ app.js:7
k @ app.js:7
D.navigate @ app.js:7
(anonymous) @ app.js:7
(anonymous) @ app.js:7
(anonymous) @ app.js:7
value @ app.js:7
_ @ app.js:7
(anonymous) @ app.js:7
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What does it mean?

Edit: It looks like this might be what is going on: