Profiles model is now a singleton, to prevent the agressive garbage collecting to break stuff

CURA-2826

cura/CuraApplication.py

@@ -549,7 +549,7 @@ class CuraApplication(QtApplication):
         qmlRegisterType(cura.Settings.ExtrudersModel, "Cura", 1, 0, "ExtrudersModel")
 
         qmlRegisterType(cura.Settings.ContainerSettingsModel, "Cura", 1, 0, "ContainerSettingsModel")
-        qmlRegisterType(cura.Settings.ProfilesModel, "Cura", 1, 0, "ProfilesModel")
+        qmlRegisterSingletonType(cura.Settings.ProfilesModel, "Cura", 1, 0, "ProfilesModel", cura.Settings.ProfilesModel.createProfilesModel)
         qmlRegisterType(cura.Settings.QualityAndUserProfilesModel, "Cura", 1, 0, "QualityAndUserProfilesModel")
         qmlRegisterType(cura.Settings.UserProfilesModel, "Cura", 1, 0, "UserProfilesModel")
         qmlRegisterType(cura.Settings.MaterialSettingsVisibilityHandler, "Cura", 1, 0, "MaterialSettingsVisibilityHandler")

cura/Settings/ProfilesModel.py

@@ -25,6 +25,21 @@ class ProfilesModel(InstanceContainersModel):
         Application.getInstance().getMachineManager().activeStackChanged.connect(self._update)
         Application.getInstance().getMachineManager().activeMaterialChanged.connect(self._update)
 
+    # Factory function, used by QML
+    @staticmethod
+    def createProfilesModel(engine, js_engine):
+        return ProfilesModel.getInstance()
+
+    ##  Get the singleton instance for this class.
+    @classmethod
+    def getInstance(cls):
+        # Note: Explicit use of class name to prevent issues with inheritance.
+        if ProfilesModel.__instance is None:
+            ProfilesModel.__instance = cls()
+        return ProfilesModel.__instance
+
+    __instance = None
+
     ##  Fetch the list of containers to display.
     #
     #   See UM.Settings.Models.InstanceContainersModel._fetchInstanceContainers().

resources/qml/Menus/ProfileMenu.qml

@@ -13,7 +13,7 @@ Menu
 
     Instantiator
     {
-        model: Cura.ProfilesModel { }
+        model: Cura.ProfilesModel 
 
         MenuItem
         {