Let app.get_viewer() fallback to ID when reference is None (#2156)

* Add test that would fail on main currently

* Let app.get_viewer() auto fallback
on using ID when viewer has no reference.

jdaviz/app.py

@@ -572,25 +572,24 @@ def get_viewer(self, viewer_reference):
 
         Notes
         -----
-            This is only used in cases where the viewers have been pre-defined
-            in the configuration file. Otherwise, viewers are not stored via
-            reference.
+        If viewer does not have a reference, it is going to try to look
+        up the viewer using the given reference as ID.
 
         Parameters
         ----------
         viewer_reference : str
             The reference to the viewer defined with the ``reference`` key
-            in the yaml configuration file.
+            in the YAML configuration file.
 
         Returns
         -------
-        `~glue_jupyter.bqplot.common.BqplotBaseView`
+        viewer : `~glue_jupyter.bqplot.common.BqplotBaseView`
             The viewer class instance.
         """
-        return self._viewer_by_reference(viewer_reference)
+        return self._viewer_by_reference(viewer_reference, fallback_to_id=True)
 
     def get_viewer_by_id(self, vid):
-        """Like :meth:`get_viewer` but use ID instead of reference name.
+        """Like :meth:`get_viewer` but use ID directly instead of reference name.
         This is useful when reference name is `None`.
         """
         return self._viewer_store.get(vid)
@@ -1460,7 +1459,7 @@ def find_viewer_item(stack_items):
 
         return viewer_item
 
-    def _viewer_by_reference(self, reference):
+    def _viewer_by_reference(self, reference, fallback_to_id=False):
         """
         Viewer instance by reference defined in the yaml configuration file.
 
@@ -1476,6 +1475,9 @@ def _viewer_by_reference(self, reference):
         """
         viewer_item = self._viewer_item_by_reference(reference)
 
+        if viewer_item is None and fallback_to_id:
+            return self._viewer_by_id(reference)
+
         return self._viewer_store[viewer_item['id']]
 
     def _viewer_item_by_reference(self, reference):

jdaviz/configs/imviz/tests/test_viewers.py

@@ -30,6 +30,13 @@ def test_create_destroy_viewer(imviz_helper, desired_name, actual_name):
     assert imviz_helper.app.get_viewer_ids() == ['imviz-0']
 
 
+def test_get_viewer_created(imviz_helper):
+    # This viewer has no reference but has ID.
+    viewer1 = imviz_helper.create_image_viewer()
+    viewer2 = imviz_helper.app.get_viewer('imviz-1')
+    assert viewer1 is viewer2
+
+
 def test_destroy_viewer_invalid(imviz_helper):
     assert imviz_helper.app.get_viewer_ids() == ['imviz-0']