Fix Transformer on problems created pre-formal mode
Fix Transformer on problems created pre-formal mode

Summary:
The mode for the Transformer was undefined when rendered from json,
since the defaultProps were only in the editor.

Test Plan: testrender.html with old problem, verify buttons work and no
js errors

Auditors: alex

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