return None if group does not exist in inventory groups

lib/ansible/inventory/__init__.py

@@ -457,7 +457,10 @@ class Inventory(object):
         return matching_host
 
     def get_group(self, groupname):
-        return self.groups[groupname]
+        if groupname in self.groups:
+            return self.groups[groupname]
+        else:
+            return None
 
     def get_group_variables(self, groupname, update_cached=False, vault_password=None):
         if groupname not in self._vars_per_group or update_cached:

lib/ansible/plugins/strategy/__init__.py

@@ -356,10 +356,10 @@ class StrategyBase:
             new_group.add_host(new_host)
 
             # add this host to the group cache
-            if self._inventory._groups_list is not None:
-                if group_name in self._inventory._groups_list:
-                    if new_host.name not in self._inventory._groups_list[group_name]:
-                        self._inventory._groups_list[group_name].append(new_host.name)
+            if self._inventory.groups is not None:
+                if group_name in self._inventory.groups:
+                    if new_host not in self._inventory.get_group(group_name).hosts:
+                        self._inventory.get_group(group_name).hosts.append(new_host.name)
 
         # clear pattern caching completely since it's unpredictable what
         # patterns may have referenced the group