Initialise a dict in a mutually circular type

I need to define two mutually circular types. I have read this issue, which suggests using a forward declaration. That works. However, I also need to contain the types in Dicts, which must be initialised.

abstract type AbstractB end

mutable struct A
  b::AbstractB
  A() = new()
end

mutable struct B <: AbstractB
  a::A
  adict::Dict{String, A}
  B() = new()
end

a = A()
b = B()

# This works
a.b = b
b.a = a

# But this gives an error, because the dictionary wasn't initialised.
b.adict["1"] = a
# ERROR: UndefRefError: access to undefined reference

This time, I use an inner constructor to initialise the Dict to an empty dictionary. It seems to recursively call the circular types, even though the dictionary is empty.

# NEW SESSION =========================

# This time, initialize the dictionary with an empty dictionary.

abstract type AbstractB end

mutable struct A
  b::AbstractB
  stringdict::Dict{String, AbstractB}
  A() = new(B(), Dict{String, B}())
end

mutable struct B <: AbstractB
  a::A
  adict::Dict{String, A}
  B() = new(A(), Dict{String, A}())
end

a = A() # StackOverflowError.  Why, given that the dict has zero entries?

Is there a way around this? I’d really like to use the types, because these structures will be used a lot.

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