fix(ct): fix issue #141 - error after Reverse of the relation between Concepts
fix(ct): fix issue #141 - error after Reverse of the relation between Concepts

Changes:
- add extra check before doing reverse action for `ConceptGraph`

cjlee112/socraticqs2,cjlee112/socraticqs2,cjlee112/socraticqs2,cjlee112/socraticqs2