  We study numerically the Toner-Tu field theory where the density field is
maintained constant, a limit case of "Malthusian" flocks for which the
asymptotic scaling of correlation functions in the ordered phase is known
exactly. While we confirm these scaling laws, we also show that such
constant-density flocks are metastable to the nucleation of a specific defect
configuration, and are replaced by a globally disordered phase consisting of
asters surrounded by shock lines that constantly evolves and remodels itself.
We demonstrate that the main source of disorder lies along shock lines,
rendering this active foam fundamentally different from the corresponding
equilibrium system. We thus show that in the context of active matter also, a
result obtained at all orders of perturbation theory can be superseded by
non-perturbative effects, calling for a different approach.
