Error message for WHIZARD FSR shower

[Juergen Reuter]

Reported by Jan Strube: The following SINDARIN input script

model = SM_CKM

mb = 0
process eebb = e1, E1 => b, B
?ps_fsr_active = true
?ps_isr_active = false
?hadronization_active = true
?ps_use_PYTHIA_shower = false

ps_max_n_flavors = 5
ps_mass_cutoff = 1 GeV

cuts = all M > 10 GeV [b,B]

sqrts = 250 GeV
beams = e1, E1 => isr
beam_polarization = diagonal_density(1: 1.0), diagonal_density(-1: 1.0)

integrate(eebb) { iterations = 10:50000 }
simulate(eebb) {
    n_events = 10000
}

leads to the strange error message:

STOP Bug: pdf only implemented for (anti-)proton