No one noticed this subtle bug I introduced for months!

Summary:
With versioning in the renderer, we should be referring to
this.state.widgetInfo over this.props.widgets almost
exclusively.

@joel noticed this subtle bug (that would manifest once
versioning started being more heavily used) in D11057.

Test Plan:
 * make an input-number
 * try serialization
 * try leaving it empty and check for highlighting
    * realize that this doesn't work anymore, and was
      broken before this commit. decide to debug this
      after this commit.
 * try empty/wrong/correct grading

Auditors: joel