Don't crash if the number of extruders in setting doesn't match

...with the actual number of extruders. These could misalign with Custom FDM Printer or when any of the extruders was deleted because of a configuration format error.

Contributes to issue #3570.

cura/Machines/QualityManager.py

@@ -163,7 +163,7 @@ class QualityManager(QObject):
     def _updateQualityGroupsAvailability(self, machine: "GlobalStack", quality_group_list):
         used_extruders = set()
         for i in range(machine.getProperty("machine_extruder_count", "value")):
-            if machine.extruders[str(i)].isEnabled:
+            if str(i) in machine.extruders and machine.extruders[str(i)].isEnabled:
                 used_extruders.add(str(i))
 
         # Update the "is_available" flag for each quality group.