Fix option_extruder case for per object settings panel

CURA-3814

plugins/PerObjectSettingsTool/PerObjectSettingsPanel.qml

@@ -94,6 +94,8 @@ Item {
                                         return settingComboBox
                                     case "extruder":
                                         return settingExtruder
+                                    case "optional_extruder":
+                                        return settingOptionalExtruder
                                     case "bool":
                                         return settingCheckBox
                                     case "str":
@@ -342,6 +344,13 @@ Item {
         Cura.SettingExtruder { }
     }
 
+    Component
+    {
+        id: settingOptionalExtruder
+
+        Cura.SettingOptionalExtruder { }
+    }
+
     Component
     {
         id: settingCheckBox;