Revert storing of messagebox

- This is NOT done because the original crash was reproducible - but just out of pure legacy reasons for if the error might still occur. It would be worth looking into whether the crash can still be reproduced in recent Blender versions without this logic.

openpype/tools/workfiles/app.py

@@ -544,6 +544,10 @@ class FilesWidget(QtWidgets.QWidget):
         # file on a refresh of the files model.
         self.auto_select_latest_modified = True
 
+        # Avoid crash in Blender and store the message box
+        # (setting parent doesn't work as it hides the message box)
+        self._messagebox = None
+
         files_view = FilesView(self)
 
         # Create the Files model
@@ -722,7 +726,7 @@ class FilesWidget(QtWidgets.QWidget):
         self.file_opened.emit()
 
     def save_changes_prompt(self):
-        messagebox = QtWidgets.QMessageBox(parent=self)
+        self._messagebox = messagebox = QtWidgets.QMessageBox(parent=self)
         messagebox.setWindowFlags(messagebox.windowFlags() |
                                   QtCore.Qt.FramelessWindowHint)
         messagebox.setIcon(messagebox.Warning)