If objdesign-of-var succeeds, read-object returns a value (not an error).
Theorem: valuep-of-read-object-of-objdesign-of-var
(defthm valuep-of-read-object-of-objdesign-of-var (b* ((objdes (objdesign-of-var var compst))) (implies objdes (valuep (read-object objdes compst)))))