Fixes for Stream without source

Author: philippjfr

holoviews/plotting/bokeh/element.py

@@ -855,7 +855,7 @@ def model_changed(self, model):
         update on the backend.
         """
         callbacks = [cb for cbs in self.traverse(lambda x: x.callbacks)
-                             for cb in cbs]
+                     for cb in cbs]
         stream_metadata = [stream._metadata for cb in callbacks
                            for stream in cb.streams if stream._metadata]
         return any(md['id'] == model.ref['id'] for models in stream_metadata

holoviews/plotting/bokeh/plot.py

@@ -161,7 +161,8 @@ def _construct_callbacks(self):
             streams = Stream.registry.get(source, [])
             registry = Stream._callbacks['bokeh']
             cb_classes |= {(registry[type(stream)], stream) for stream in streams
-                           if type(stream) in registry and stream.linked}
+                           if type(stream) in registry and stream.linked
+                           and stream.source is not None}
         cbs = []
         sorted_cbs = sorted(cb_classes, key=lambda x: id(x[0]))
         for cb, group in groupby(sorted_cbs, lambda x: x[0]):