Fix crash when getting signals for closed tabs

Fixes #3498

qutebrowser/browser/browsertab.py

@@ -749,6 +749,10 @@ class AbstractTab(QWidget):
 
     @pyqtSlot(bool)
     def _on_load_finished(self, ok):
+        if sip.isdeleted(self._widget):
+            # https://github.com/qutebrowser/qutebrowser/issues/3498
+            return
+
         sess_manager = objreg.get('session-manager')
         sess_manager.save_autosave()
 

qutebrowser/browser/webengine/webenginetab.py

@@ -631,6 +631,9 @@ class WebEngineTab(browsertab.AbstractTab):
 
     @pyqtSlot()
     def _restore_zoom(self):
+        if sip.isdeleted(self._widget):
+            # https://github.com/qutebrowser/qutebrowser/issues/3498
+            return
         if self._saved_zoom is None:
             return
         self.zoom.set_factor(self._saved_zoom)