Fixing small bug in activate.

If the parallelmagic extension has not been loaded, activate would
get an empty list from get_component. Now get_component returns None
in that case and activate checks for None and prints a message to
the user to load the extension.

IPython/core/iplib.py

@@ -2294,6 +2294,8 b' class InteractiveShell(Component, Magic):'
     def get_component(self, name=None, klass=None):
         """Fetch a component by name and klass in my tree."""
         c = Component.get_instances(root=self, name=name, klass=klass)
+        if len(c) == 0:
+            return None
         if len(c) == 1:
             return c[0]
         else:

IPython/kernel/multiengineclient.py

@@ -269,7 +269,11 b' class InteractiveMultiEngineClient(object):'
             print "The IPython parallel magics (%result, %px, %autopx) only work within IPython."
         else:
             pmagic = ip.get_component('parallel_magic')
+            if pmagic is not None:
             pmagic.active_multiengine_client = self
+            else:
+                print "You must first load the parallelmagic extension " \
+                      "by doing '%load_ext parallelmagic'"
                     
     def __setitem__(self, key, value):
         """Add a dictionary interface for pushing/pulling.