FIX/WIP: the input to the signal handler can be None?

Depending on order of operations the notifier can be cleared before
the callback is executed.

Not really clear on how this can be, but seeing this happen locally.

Author: tacaswell

lib/matplotlib/backends/backend_qt.py

@@ -175,8 +175,11 @@ def _may_clear_sock():
         return sn
 
     def handle_sigint(sn):
-        sn.deleteLater()
-        QtCore.QCoreApplication.sendPostedEvents(sn, QtCore.QEvent.Type.DeferredDelete)
+        if sn is not None:
+            sn.deleteLater()
+            QtCore.QCoreApplication.sendPostedEvents(
+                sn, QtCore.QEvent.Type.DeferredDelete
+            )
         if hasattr(qapp_or_eventloop, 'closeAllWindows'):
             qapp_or_eventloop.closeAllWindows()
         qapp_or_eventloop.quit()