Thanks to #16700, packages listed in dotspacemacs-additional-packages are installed even if some enabled layer has a :toggle form for that package which evals to nil. However, it seems that in this case, Spacemacs attempts to apply the layer configuration for that package, which can lead to errors (and seems like undesired beahvior anyway).

Consider a config which is like the default, except that dotspacemacs-additional-packages contains evil-swap-keys because the user wants to use it directly. dotspacemacs-swap-number-row is nil, as is the default.

However, because spacemacs-editing configures evil-swap-keys, spacemacs-editing/init-evil-swap-keys runs at startup. Since spacemacs-editing/init-evil-swap-keys (reasonably) assumes dotspacemacs-swap-number-row is non-nil, you get this error:

evil-swap-keys-swap-number-row: Wrong type argument: listp, "dotspacemacs-swap-number-row nil is not supported."