owpreprocess: fix potential crash at destruction

[markotoplak]

Issue

When signals are handled when objects are being destroyed we can bugs as in #6136.

The test in https://github.com/biolab/orange3-single-cell/actions/runs/6010124237/job/16300932397?pr=395 failed with

 test_editor_normalize_groups (tests.test_owscpreprocess.TestNormalizeEditor) ... Traceback (most recent call last):
  File "/Users/runner/work/orange3-single-cell/orange3-single-cell/.tox/orange-oldest/lib/python3.9/site-packages/Orange/widgets/data/owpreprocess.py", line 1412, in __update_size_constraint
    sh = self.flow_view.minimumSizeHint()
AttributeError: 'OWscPreprocess' object has no attribute 'flow_view'

Thanks to @elapraznik for the crash. :)

Description of changes

Check if the object is still live.

Includes

• Code changes
• Tests
• Documentation

[janezd]

Zombie objects are bug in PyQt's architecture.

Methods (except those connected to the signal destroyed) shouldn't have to check whether an attribute that is set in the constructor still exist. Hence, I would prefer that this bug is fixed in the meta class: any method that can be called on a zombie (to my and Marko's knowledge: eventFilter) should be wrapped in a method that checks whether self.__dict__ is non-empty. If non-empty, it calls the actual method, otherwise it calls just the super.

@ales-erjavec, would this be OK?

Edit: something like biolab/orange-widget-base#256. This however affects only OWPreprocess and OWBoxplot. All other eventFilters are in classes outside the OWWidget hierarchy.

[ales-erjavec]

I think this would be a better solution

diff --git a/Orange/widgets/data/owpreprocess.py b/Orange/widgets/data/owpreprocess.py
index 0d1c5de8f..872cfcd50 100644
--- a/Orange/widgets/data/owpreprocess.py
+++ b/Orange/widgets/data/owpreprocess.py
@@ -1178,6 +1178,10 @@ class OWPreprocess(widget.OWWidget, openclass=True):
 
         gui.auto_apply(self.buttonsArea, self, "autocommit")
 
+        self.__update_size_constraint_timer = QTimer(
+            self, singleShot=True, interval=0,
+        )
+        self.__update_size_constraint_timer.timeout.connect(self.__update_size_constraint)
         self._initialize()
 
     def _initialize(self):
@@ -1367,8 +1371,7 @@ class OWPreprocess(widget.OWWidget, openclass=True):
 
     def eventFilter(self, receiver, event):
         if receiver is self.flow_view and event.type() == QEvent.LayoutRequest:
-            QTimer.singleShot(0, self.__update_size_constraint)
-
+            self.__update_size_constraint_timer.start()
         return super().eventFilter(receiver, event)
 
     def storeSpecificSettings(self):