fix test - handle startup case before flux/angle unit are both populated

jdaviz/configs/specviz/plugins/unit_conversion/unit_conversion.py

@@ -175,7 +175,7 @@ def _on_glue_y_display_unit_changed(self, y_unit_str):
         if self.app.data_collection[0]:
             dc_unit = self.app.data_collection[0].get_component("flux").units
             self.angle_unit.choices = create_angle_equivalencies_list(dc_unit)
-            self.angle_unit.selected = self.angle_unit.choices[0]
+            self.angle_unit.select_default()
             self.sb_unit_selected = self._append_angle_correctly(
                 self.flux_unit.selected,
                 self.angle_unit.selected
@@ -235,6 +235,9 @@ def _on_flux_or_angle_unit_changed(self, msg):
             return
         if not self.flux_unit.choices:
             return
+        if self.flux_unit.selected == '' or self.angle_unit.selected == '':
+            # wait until flux AND angle unit are both populated during init
+            return
 
         # various plugins are listening for changes in either flux or sb and
         # need to be able to filter messages accordingly, so broadcast both when