StatelessBFS assumes that children are indexable

Iterating over StatelessBFS calls getindex on the object returned by children():

julia> collect(StatelessBFS(n))
ERROR: MethodError: no method matching getindex(::VectorTreeChildren, ::Int64)
Stacktrace:
 [1] getdescendant
   @ ~/.julia/packages/AbstractTrees/kBTzE/src/indexing.jl:29 [inlined]
 [2] iterate(ti::StatelessBFS{VectorTreeNode}, ind::Vector{Any})
   @ AbstractTrees ~/.julia/packages/AbstractTrees/kBTzE/src/iteration.jl:418

As far as I understand, the default for ChildIndexing is NonIndexedChildren, which should imply that it shouldn't be necessary to implement indexing for children? The other iterators (*OrderDFS, Leaves) work fine with my type.

This should work as an MWE:

struct VectorTreeNode
    children :: Vector{Any}
    VectorTreeNode(cs) = (cs isa Vector) ? new(cs) : new([])
end
struct VectorTreeChildren
    node :: VectorTreeNode
end
Base.IteratorSize(::Type{VectorTreeChildren}) = Base.SizeUnknown()
function Base.iterate(it::VectorTreeChildren, state = 1)
    if state <= length(it.node.children)
        (VectorTreeNode(it.node.children[state]), state + 1)
    else
        nothing
    end
end
using AbstractTrees
AbstractTrees.children(n::VectorTreeNode) = VectorTreeChildren(n)
n = VectorTreeNode([1,[2,3],4])
collect(StatelessBFS(n))

Note: there is also #15 which seems to be a similar issue, but is quite outdated I think -- Leaves works and the indexability trait (ChildIndexing) exists now.

The failure is in indexing.jl:29, reached from iteration.jl:418; start by reading getdescendant and StatelessBFS iteration alongside ChildIndexing and NonIndexedChildren. Run the supplied VectorTreeNode MWE and verify StatelessBFS can collect non-indexable children without regressing the working DFS and Leaves iterators.