Fix AttributeError on config changes on Ubuntu.

Fixes #390.

qutebrowser/browser/webview.py

@@ -20,6 +20,7 @@
 """The main browser widgets."""
 
 import itertools
+import functools
 
 from PyQt5.QtCore import pyqtSignal, pyqtSlot, Qt, QTimer, QUrl
 from PyQt5.QtWidgets import QApplication
@@ -96,7 +97,13 @@ class WebView(QWebView):
         self._zoom = None
         self._has_ssl_errors = False
         self.init_neighborlist()
-        objreg.get('config').changed.connect(self.init_neighborlist)
+        config = objreg.get('config')
+        config.changed.connect(self.init_neighborlist)
+        # For some reason, this signal doesn't get disconnected automatically
+        # when the WebView is destroyed on older PyQt versions.
+        # See https://github.com/The-Compiler/qutebrowser/issues/390
+        self.destroyed.connect(functools.partial(
+            config.changed.disconnect, self.init_neighborlist))
         self._cur_url = None
         self.cur_url = QUrl()
         self.progress = 0