Internal error inverting element of tower ring constructed with toField

Reported by Nick in the Google group (https://groups.google.com/g/macaulay2/c/8VY3X-gvkVE/m/-bhwUZ9wAAAJ):

```m2
i1 : F = toField(QQ[i]/(i^2 + 1));

i2 : K = toField(F[r]/(r^4 - 2));

i3 : r^-1
--internal error: ring element gcd computation failed
Aborted (core dumped)
```

Reproduce the reported Macaulay2 session with the nested toField constructions and the r^-1 expression. Investigate the ring-element gcd computation involved in inversion; done means the expression completes without an internal error or abort and its result is validated against the constructed tower ring.

Written by the indexing model from the issue text.