No longer store strong references to parent object in guiHelper.BoxSi…

…zerHelper to decrease likelihood of circular references in NVDA's GUI (nvaccess#16079)

Related to nvaccess#16019

Summary of the issue:
BoxSizerHelper class stores a strong reference to its parent. When it is mistakenly assigned to a member of its parent a circular reference is created making it impossible for Python's garbage collector to destroy the dialog when it goes out of scope.

Description of user facing changes
Should not be noticeable.

Description of development approach
BoxSizerHelper now stores a weak reference to its parent.

source/gui/guiHelper.py

@@ -1,6 +1,5 @@
-# -*- coding: UTF-8 -*-
 # A part of NonVisual Desktop Access (NVDA)
-# Copyright (C) 2016-2023 NV Access Limited
+# Copyright (C) 2016-2024 NV Access Limited, Łukasz Golonka
 # This file is covered by the GNU General Public License.
 # See the file COPYING for more details.
 
@@ -44,6 +43,7 @@ def __init__(self, parent):
 	...
 """
 from contextlib import contextmanager
+import weakref
 from typing import (
 	Generic,
 	Optional,
@@ -314,7 +314,7 @@ def __init__(
 			@type orientation: wx.HORIZONTAL or wx.VERTICAL
 			@param sizer: the sizer to use rather than constructing one.
 		"""
-		self._parent = parent
+		self._parentRef = weakref.ref(parent)
 		self.hasFirstItemBeenAdded = False
 		if orientation and sizer:
 			raise ValueError("Supply either orientation OR sizer. Not both.")
@@ -384,7 +384,7 @@ def addLabeledControl(
 			Relies on guiHelper.LabeledControlHelper and thus guiHelper.associateElements, and therefore inherits any
 			limitations from there.
 		"""
-		parent = self._parent
+		parent = self._parentRef()
 		if isinstance(self.sizer, wx.StaticBoxSizer):
 			parent = self.sizer.GetStaticBox()
 		labeledControl = LabeledControlHelper(parent, labelText, wxCtrlClass, **kwargs)
@@ -422,11 +422,11 @@ def addDialogDismissButtons(
 		elif isinstance(buttons, (wx.Sizer, wx.Button)):
 			toAdd = buttons
 		elif isinstance(buttons, int):
-			toAdd = self._parent.CreateButtonSizer(buttons)
+			toAdd = self._parentRef().CreateButtonSizer(buttons)
 		else:
 			raise NotImplementedError("Unknown type: {}".format(buttons))
 		if separated:
-			parentBox = self._parent
+			parentBox = self._parentRef()
 			if isinstance(self.sizer, wx.StaticBoxSizer):
 				parentBox = self.sizer.GetStaticBox()
 			self.addItem(wx.StaticLine(parentBox), flag=wx.EXPAND)