Even though I am unable to repro this issue locally or on a test instance (before/after rebuild), I just pushed a fix that will solve no implicit conversion of nil into String issue:
https://github.com/discourse/discourse/commit/2a2b0d780346542d8a8389ab5cc09110269c7149
The above fix will prevent raising exception and will ensure that digest email is sent even if @anchor_color is nil. Anchor color will fallback to default color (006699).
This is not a definitive fix, as I am not sure why Sidekiq is not loading @anchor_color's value in some cases.