Fix loading GlobalStacks on different thread

The findContainerStacks() will list all container stacks and lazily load them. However this lazy loading is done on the thread that's calling it. The lazy loading will create GlobalStack objects, which are QObjects. The QML code then can't access those QObjects because they are created on different threads. So now instead we'll do the find query on the main thread but all the rest on the background thread.

Contributes to issue CURA-6973.

cura/Machines/ContainerTree.py

@@ -16,6 +16,7 @@ import time
 if TYPE_CHECKING:
     from cura.Machines.QualityGroup import QualityGroup
     from cura.Machines.QualityChangesGroup import QualityChangesGroup
+    from UM.Settings.ContainerStack import ContainerStack
 
 
 ##  This class contains a look-up tree for which containers are available at
@@ -75,7 +76,8 @@ class ContainerTree:
 
     ##  Ran after completely starting up the application.
     def _onStartupFinished(self):
-        JobQueue.getInstance().add(self.MachineNodeLoadJob(self))
+        currently_added = ContainerRegistry.getInstance().findContainerStacks()  # Find all currently added global stacks.
+        JobQueue.getInstance().add(self.MachineNodeLoadJob(self, currently_added))
 
     ##  Dictionary-like object that contains the machines.
     #
@@ -132,16 +134,19 @@ class ContainerTree:
         #   \param tree_root The container tree instance. This cannot be
         #   obtained through the singleton static function since the instance
         #   may not yet be constructed completely.
-        def __init__(self, tree_root: "ContainerTree"):
+        #   \param container_stacks All of the stacks to pre-load the container
+        #   trees for. This needs to be provided from here because the stacks
+        #   need to be constructed on the main thread because they are QObject.
+        def __init__(self, tree_root: "ContainerTree", container_stacks: List["ContainerStack"]):
             self.tree_root = tree_root
+            self.container_stacks = container_stacks
             super().__init__()
 
         ##  Starts the background task.
         #
         #   The ``JobQueue`` will schedule this on a different thread.
         def run(self) -> None:
-            currently_added = ContainerRegistry.getInstance().findContainerStacks()  # Find all currently added global stacks.
-            for stack in currently_added:
+            for stack in self.container_stacks:
                 time.sleep(0.5)
                 if not isinstance(stack, GlobalStack):
                     continue