Handle event_target() being None

As a stop-gap solution for #3888

qutebrowser/browser/browsertab.py

@@ -724,7 +724,13 @@ class AbstractTab(QWidget):
         if getattr(evt, 'posted', False):
             raise utils.Unreachable("Can't re-use an event which was already "
                                     "posted!")
+
         recipient = self.event_target()
+        if recipient is None:
+            # https://github.com/qutebrowser/qutebrowser/issues/3888
+            log.webview.warning("Unable to find event target!")
+            return
+
         evt.posted = True
         QApplication.postEvent(recipient, evt)
 

qutebrowser/browser/webengine/webenginetab.py

@@ -1104,6 +1104,4 @@ class WebEngineTab(browsertab.AbstractTab):
         self.predicted_navigation.connect(self._on_predicted_navigation)
 
     def event_target(self):
-        fp = self._widget.focusProxy()
-        assert fp is not None
-        return fp
+        return self._widget.focusProxy()