Fix out-of-order cancellation (#450)

* Fix out-of-order cancellation

* Add changelog entry

docs/source/changes.rst

@@ -169,6 +169,8 @@ Documentation
 * The ``pi_iterations`` example has been fixed to give correct counts.
   Previously it was giving incorrect results as a result of NumPy integer
   overflow.
+* The ``prime_counting`` example has been fixed to avoid an occasional
+  ``AttributeError`` under unusual timing conditions.
 
 
 Release 0.2.0

docs/source/guide/examples/prime_counting.py

@@ -56,9 +56,10 @@ def cancel(self):
         """
         Cancel the running future when the cancel button is pressed.
         """
-        self.future.cancel()
         self._cancel_button.setEnabled(False)
-        self.message = "Cancelling\N{HORIZONTAL ELLIPSIS}"
+        cancelled = self.future.cancel()
+        if cancelled:
+            self.message = "Cancelling\N{HORIZONTAL ELLIPSIS}"
 
     # Private traits ##########################################################
 
@@ -83,7 +84,7 @@ def _create_cancel_button(self, parent):
             "Cancel", QtGui.QDialogButtonBox.RejectRole
         )
         self._cancel_button.setDefault(True)
-        buttons.rejected.connect(self.cancel)
+        buttons.rejected.connect(self.cancel, type=QtCore.Qt.QueuedConnection)
         return buttons
 
     def _create_message(self, dialog, layout):
@@ -127,7 +128,6 @@ def _report_progress(self, event):
 
     @observe("future:done")
     def _respond_to_completion(self, event):
-        self.future = None
         self.close()