Fix but with radio widget's readonly mode.
Fix but with radio widget's readonly mode.

Test Plan:
Load persues exercise with Joel's provided JSON and obs3rve no error.

ariabuckles/perseus,chirilo/perseus,abe4mvp/perseus,sachgits/perseus,abe4mvp/perseus,learningequality/perseus,alexristich/perseus,daukantas/perseus,alexristich/perseus,sachgits/perseus,chirilo/perseus,daukantas/perseus,iamchenxin/perseus,iamchenxin/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus