Catch failed waveform generation cases.

bilby/gw/psi4_waveform_generator.py

@@ -130,6 +130,8 @@ class Psi4_WaveformGenerator(WaveformGenerator):
             FD_psi4 = self._FD_psi4_from_FD_waveform(model_waveform) \
                     if is_FD_model else \
                     self._FD_psi4_from_TD_waveform(model_waveform)
+        elif model_waveform is None:
+            FD_psi4 = None
         else:
             FD_psi4 = dict()
             for key in model_waveform:
@@ -154,6 +156,8 @@ class Psi4_WaveformGenerator(WaveformGenerator):
 
         if isinstance(FD_psi4, np.ndarray):
             TD_psi4 = utils.infft(FD_psi4, self.sampling_frequency)
+        elif FD_psi4 is None:
+            TD_psi4 = None
         else:
             TD_psi4 = dict()
             for key in FD_psi4: