Fix uncaught exception on channel list key navigation

After searching, the channel list treeview sometimes does not have a
selected item, leading to an invalid iterator when navigating the list
with arrow keys.

Select the first episode item in the TreeView model in this case.

src/gpodder/gtkui/main.py

@@ -717,14 +717,17 @@ class gPodder(BuilderWidget, dbus.service.Object):
             elif event.keyval in (Gdk.KEY_Up, Gdk.KEY_Down):
                 # If section markers exist in the treeview, we want to
                 # "jump over" them when moving the cursor up and down
-                selection = self.treeChannels.get_selection()
-                model, it = selection.get_selected()
-
                 if event.keyval == Gdk.KEY_Up:
                     step = -1
                 else:
                     step = 1
 
+                selection = self.treeChannels.get_selection()
+                model, it = selection.get_selected()
+                if it is None:
+                    it = model.get_iter_first()
+                    step = 1
+
                 path = model.get_path(it)
                 while True:
                     path = (path[0] + step,)