To Reproduce
Steps to reproduce the behavior:

1. Call NthRoot
2. Type 1
3. Type 2
4. Type -
5. Type x
6. Click near x
7. Exception

Expected behavior
Exception is not thrown.

Additional context
Atoms inside of a degree are "fused", so "12 - x" is stored only as three atoms:

{Ordinary 12}
{Binary Operator −}
{Ordinary 𝑥}

And here is lines in the MathListIndexToStringIndex function from where exception comes:

if (mlIndex >= self.Atoms.Length)
        throw ArgOutOfRange($"The index is not in the range {self.Atoms.Length}", mlIndex, nameof(mlIndex));