Qt5->Qt6: Fix another stack-overflow error for newer Python/Qt.

part of CURA-8591

cura/API/__init__.py

@@ -34,12 +34,13 @@ class CuraAPI(QObject):
             raise RuntimeError("Tried to create singleton '{class_name}' more than once.".format(class_name = CuraAPI.__name__))
         if application is None:
             raise RuntimeError("Upon first time creation, the application must be set.")
-        cls.__instance = super(CuraAPI, cls).__new__(cls)
+        instance = super(CuraAPI, cls).__new__(cls)
         cls._application = application
-        return cls.__instance
+        return instance
 
     def __init__(self, application: Optional["CuraApplication"] = None) -> None:
         super().__init__(parent = CuraAPI._application)
+        CuraAPI.__instance = self
 
         self._account = Account(self._application)
 

cura/CuraApplication.py

@@ -217,7 +217,7 @@ class CuraApplication(QtApplication):
 
         self._quality_profile_drop_down_menu_model = None
         self._custom_quality_profile_drop_down_menu_model = None
-        self._cura_API = None
+        self._cura_API = CuraAPI(self)
 
         self._physics = None
         self._volume = None
@@ -837,7 +837,7 @@ class CuraApplication(QtApplication):
         self._setting_visibility_presets_model = SettingVisibilityPresetsModel(self.getPreferences(), parent = self)
 
         # Initialize Cura API
-        self._cura_API = self.getCuraAPI()
+        self._cura_API.initialize()
         self.processEvents()
         self._output_device_manager.start()
         self._welcome_pages_model.initialize()
@@ -1114,9 +1114,6 @@ class CuraApplication(QtApplication):
         return self._custom_quality_profile_drop_down_menu_model
 
     def getCuraAPI(self, *args, **kwargs) -> "CuraAPI":
-        if not self._cura_API:
-            self._cura_API = CuraAPI(self)
-            self._cura_API.initialize()
         return self._cura_API
 
     def registerObjects(self, engine):