Fix completion selection bug.

Fix the issue where pressing `o<esc>o` would show a url completion
dialog where attempting to <Tab> select items would do nothing but show
a Qt warning.

The fix is to ensure we set _last_completion_func to None whenever we
clear completion (there was a case I missed).

It also ensures we always delete the old model and adds a safety to
prevent deleting an in-use model is set_model is called with the current
model.

qutebrowser/completion/completer.py

@@ -211,6 +211,7 @@ class Completer(QObject):
             # FIXME complete searches
             # https://github.com/qutebrowser/qutebrowser/issues/32
             completion.set_model(None)
+            self._last_completion_func = None
             return
 
         before_cursor, pattern, after_cursor = self._partition()

qutebrowser/completion/completionwidget.py

@@ -269,17 +269,18 @@ class CompletionView(QTreeView):
         Args:
             model: The model to use.
         """
+        if self.model() is not None and model is not self.model():
+            self.model().deleteLater()
+            self.selectionModel().deleteLater()
+
+        self.setModel(model)
+
         if model is None:
             self._active = False
             self.hide()
             return
 
-        if self.model() is not None:
-            self.model().deleteLater()
-            self.selectionModel().deleteLater()
-
         model.setParent(self)
-        self.setModel(model)
         self._active = True
         self._maybe_show()