Inserting Latex expressions into the expression tree is breaking

[rwmorton]

I have a keyboard that inserts latex into the MathQuill field - so if I press sqrt for example it will insert \sqrt. It works fine if the MathQuill field is empty.

But if there is at least one character in the field and I "insert" latex from the keyboard, eg.

It breaks. Here is the error:

And here is the source:

Note that field itself is a Vue ref - hence the field.value ...