Fix "undefined" new hints.

We were accessing `this.props.hint.content` when `hint` was just `{}`.

Test Plan:
Add, like, 5 hints to a problem. None of them say "undefined". Type some
nonsense in one of the hints and watch it show up in the preview on the
right.

Auditors: eater
Focus.

Test Plan:
Switch problem type to "text input (number)" - the input is
automagically focused.

Auditors: alex