Merge pull request #4802 from mtryan83/switch_to_ds_first

Fix bug where race condition results in incorrect fields categorization when computing particle_trajectories

yt/data_objects/particle_trajectories.py

@@ -234,7 +234,7 @@ def _get_data(self, fields):
             fds[field] = dd_first._determine_fields(field)[0]
             if field not in self.particle_fields:
                 ftype = fds[field][0]
-                if ftype in self.data_series[0].particle_types:
+                if ftype in ds_first.particle_types:
                     self.particle_fields.append(field)
                     new_particle_fields.append(field)