Check that the cursor object has reference to an actual DOM element
in its moveUpDown method.

This is due to a bug observed when calling cursor.moveDown via the softkey
command.

Using the touch device / keyboard interface (so cursor commands are
called programmatically):
- Create a new fraction in the editor
- Press a number to add as numerator (top part)
- Attempt to moveDown (clicking soft keyboard down key) to the denominator (bottom part).