Replace getUsedExtruderNumbers with the faster alternative

CURA-7106

plugins/CuraEngineBackend/StartSliceJob.py

@@ -352,8 +352,7 @@ class StartSliceJob(Job):
 
         result = {}
         for key in stack.getAllKeys():
-            value = stack.getProperty(key, "value")
-            result[key] = value
+            result[key] = stack.getProperty(key, "value")
             Job.yieldThread()
 
         result["print_bed_temperature"] = result["material_bed_temperature"] # Renamed settings.
@@ -454,11 +453,10 @@ class StartSliceJob(Job):
         print_temperature_settings = ["material_print_temperature", "material_print_temperature_layer_0", "default_material_print_temperature", "material_initial_print_temperature", "material_final_print_temperature", "material_standby_temperature"]
         pattern = r"\{(%s)(,\s?\w+)?\}" % "|".join(print_temperature_settings) # match {setting} as well as {setting, extruder_nr}
         settings["material_print_temp_prepend"] = re.search(pattern, start_gcode) == None
+
         # Replace the setting tokens in start and end g-code.
         # Use values from the first used extruder by default so we get the expected temperatures
-        initial_extruder_stack = CuraApplication.getInstance().getExtruderManager().getUsedExtruderStacks()[0]
-        initial_extruder_nr = initial_extruder_stack.getProperty("extruder_nr", "value")
-
+        initial_extruder_nr = CuraApplication.getInstance().getExtruderManager().getInitialExtruderNr()
         settings["machine_start_gcode"] = self._expandGcodeTokens(settings["machine_start_gcode"], initial_extruder_nr)
         settings["machine_end_gcode"] = self._expandGcodeTokens(settings["machine_end_gcode"], initial_extruder_nr)