Get the next derivation index for keychain. The next index is the index after the last revealed
derivation index.
The second field in the returned tuple represents whether the next derivation index is new.
There are two scenarios where the next derivation index is reused (not new):
@@ -184,15 +181,15 @@ There are two scenarios where the next derivation index is reused (not new):
Not checking the second field of the tuple may result in address reuse.
Returns None if the provided keychain doesnât exist.
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