nullKeyEntryIndices

public int[] nullKeyEntryIndices { get; }

The Inspector keeps a row with a null key so a user can assign a valid key to it, but a null key can't exist in the runtime Dictionary, so Unity excludes these rows from the live dictionary. The indices are positions in the dictionary's underlying serialized array of key/value pairs, and are stable across Inspector re-sorts. This array is empty when the dictionary has no null-key rows.