The printer to override was not being set initially, causing file loading to fail when not selecting a printer in the MachineSelector.

Fix is to set it on initialization from the WorkspaceReader

CURA-9424

plugins/3MFReader/ThreeMFWorkspaceReader.py

@@ -600,6 +600,7 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
         self._dialog.setActiveMode(active_mode)
         self._dialog.setUpdatableMachines(updatable_machines)
         self._dialog.setMachineName(machine_name)
+        self._dialog.setMachineToOverride(global_stack_id)
         self._dialog.setMaterialLabels(material_labels)
         self._dialog.setMachineType(machine_type)
         self._dialog.setExtruders(extruders)