Fix: Suppress AttributeError tracebacks during widget teardown on Python 3.14 / Qt6

During widget destruction, Qt still delivers events and calls size-hint
methods on QwtPlot objects whose Python __dict__ has already been cleared,
so accessing private instance attributes raises an uncaught AttributeError.

This produced noisy "Unhandled exception" tracebacks at pytest teardown for
several subclasses (BasePlot, LevelsHistogram, XCrossSectionPlot,
YCrossSectionPlot), reporting 'object has no attribute _QwtPlot__data'.

* [FIX] : Guard access to self.__data.canvas in QwtPlot.eventFilter with
  try/except AttributeError, delegating to QFrame.eventFilter on failure
* [FIX] : Guard access to self.__data.layout in QwtPlot.minimumSizeHint
  with try/except AttributeError, returning an empty QSize() on failure

Author: dappham-CODRA

qwt/plot.py

@@ -1007,7 +1007,12 @@ def event(self, event):
         return QFrame.event(self, event)
 
     def eventFilter(self, obj, event):
-        if obj is self.__data.canvas:
+        try:
+            canvas = self.__data.canvas
+        except AttributeError:
+            # The plot is being destroyed: Python attributes already cleared
+            return QFrame.eventFilter(self, obj, event)
+        if obj is canvas:
             if event.type() == QEvent.Resize:
                 self.updateCanvasMargins()
             elif event.type() == 178:  # QEvent.ContentsRectChange:
@@ -1195,7 +1200,12 @@ def minimumSizeHint(self):
         """
         :return: Return a minimum size hint
         """
-        hint = self.__data.layout.minimumSizeHint(self)
+        try:
+            layout = self.__data.layout
+        except AttributeError:
+            # The plot is being destroyed: Python attributes already cleared
+            return QSize()
+        hint = layout.minimumSizeHint(self)
         hint += QSize(2 * self.frameWidth(), 2 * self.frameWidth())
         return hint