fallback for object-cache

* mosviz tests are failing on GitHub CI, but not locally, because not found in cache.  Could just be a different order of callbacks being processed, so this should result in the same thing and force the cache to be populated

jdaviz/configs/imviz/plugins/coords_info/coords_info.py

@@ -391,7 +391,7 @@ def _spectrum_viewer_update(self, viewer, x, y):
         def _cursor_fallback():
             statistic = getattr(viewer.state, 'function', None)
             cache_key = (viewer.state.layers[0].layer.label, statistic)
-            sp = self.app._get_object_cache[cache_key]
+            sp = self.app._get_object_cache.get(cache_key, viewer.data()[0])
             self._dict['axes_x'] = x
             self._dict['axes_x:unit'] = sp.spectral_axis.unit.to_string()
             self._dict['axes_y'] = y