Stop listening to its children before the template editor is destroyed

Closing a title block template editor crashed it. The editor deletes
itself on close; QWidget's destructor then deletes its children, and
some of them signal the editor as they go:

- the scene deselects its cells, so the view emits
  selectedCellsChanged() to the editor's slot of the same name (when a
  cell was selected);
- the undo stack is cleared, so it emits cleanChanged() to
  updateEditorTitle() (when the template was edited).

Both slots then run on an object whose destructor has already run. A
Debug build aborts ("Called object is not of the correct type (class
destructor may have already run)"); a release build calls a member
function of a destroyed object.

Disconnect every child of the editor from it in the editor's
destructor, before the base destructors delete the children.

No file-format change, no change while the editor is open.

Tests: none automated; the editor needs the whole application.
Checked by hand (see the PR).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Beat Hangartner <beat@hangartners.ch>

sources/titleblock/qettemplateeditor.cpp

@@ -55,7 +55,17 @@ QETTitleBlockTemplateEditor::QETTitleBlockTemplateEditor(QWidget *parent) :
 	Destructor
 */
 QETTitleBlockTemplateEditor::~QETTitleBlockTemplateEditor()
-{}
+{
+		// QWidget's destructor deletes the children, and some of them
+		// signal this editor as they go: the scene deselects its cells (the
+		// view emits selectedCellsChanged()), the undo stack is cleared
+		// (cleanChanged()). By then this editor is already destroyed, so
+		// stop listening to all of its children first.
+	const auto children = findChildren<QObject *>();
+	for (QObject *child : children) {
+		disconnect(child, nullptr, this, nullptr);
+	}
+}
 
 /**
 	@return the location of the currently edited template