Turn MaterialManagementModel into a singleton

Just like MaterialManager used to be. There can be only one instance of the page then.
This prevents a crash when Qt deletes the QObject because it's no longer used in the page when you close the preferences screen. But when you open it again it doesn't construct a new one. Now there is always one instance so that's not a problem any more. Also it allows other pages to access this item.

Contributes to issue CURA-6600.

cura/CuraApplication.py

@@ -222,6 +222,7 @@ class CuraApplication(QtApplication):
         self._machine_error_checker = None
 
         self._machine_settings_manager = MachineSettingsManager(self, parent = self)
+        self._material_management_model = MaterialManagementModel()
 
         self._discovered_printer_model = DiscoveredPrintersModel(self, parent = self)
         self._first_start_machine_actions_model = FirstStartMachineActionsModel(self, parent = self)
@@ -976,6 +977,10 @@ class CuraApplication(QtApplication):
     def getMachineActionManager(self, *args):
         return self._machine_action_manager
 
+    @pyqtSlot(result = QObject)
+    def getMaterialManagementModel(self):
+        return self._material_management_model
+
     def getSimpleModeSettingsManager(self, *args):
         if self._simple_mode_settings_manager is None:
             self._simple_mode_settings_manager = SimpleModeSettingsManager()
@@ -1055,7 +1060,7 @@ class CuraApplication(QtApplication):
         qmlRegisterType(GenericMaterialsModel, "Cura", 1, 0, "GenericMaterialsModel")
         qmlRegisterType(MaterialBrandsModel, "Cura", 1, 0, "MaterialBrandsModel")
         qmlRegisterType(QualityManagementModel, "Cura", 1, 0, "QualityManagementModel")
-        qmlRegisterType(MaterialManagementModel, "Cura", 1, 5, "MaterialManagementModel")
+        qmlRegisterSingletonType(MaterialManagementModel, "Cura", 1, 5, "MaterialManagementModel", self.getMaterialManagementModel)
 
         qmlRegisterType(DiscoveredPrintersModel, "Cura", 1, 0, "DiscoveredPrintersModel")
 

resources/qml/Preferences/Materials/MaterialsPage.qml

@@ -18,6 +18,7 @@ Item
     property var currentItem: null
 
     property var materialManager: CuraApplication.getMaterialManager()
+    property var materialManagementModel: CuraApplication.getMaterialManagementModel()
 
     property var hasCurrentItem: base.currentItem != null
     property var isCurrentItemActivated:
@@ -42,11 +43,6 @@ Item
         name: "cura"
     }
 
-    Cura.MaterialManagementModel
-    {
-        id: materialManagement
-    }
-
     function resetExpandedActiveMaterial()
     {
         materialListView.expandActiveMaterial(active_root_material_id)
@@ -152,7 +148,7 @@ Item
             id: removeMenuButton
             text: catalog.i18nc("@action:button", "Remove")
             iconName: "list-remove"
-            enabled: base.hasCurrentItem && !base.currentItem.is_read_only && !base.isCurrentItemActivated && materialManagement.canMaterialBeRemoved(base.currentItem.container_node)
+            enabled: base.hasCurrentItem && !base.currentItem.is_read_only && !base.isCurrentItemActivated && base.materialManagementModel.canMaterialBeRemoved(base.currentItem.container_node)
 
             onClicked:
             {
@@ -302,7 +298,7 @@ Item
         {
             // Set the active material as the fallback. It will be selected when the current material is deleted
             base.newRootMaterialIdToSwitchTo = base.active_root_material_id
-            base.materialManagement.removeMaterial(base.currentItem.container_node);
+            base.materialManagementModel.removeMaterial(base.currentItem.container_node);
         }
     }
 

resources/qml/Preferences/Materials/MaterialsView.qml

@@ -23,6 +23,7 @@ TabView
     property real secondColumnWidth: (width * 0.40) | 0
     property string containerId: ""
     property var materialPreferenceValues: UM.Preferences.getValue("cura/material_settings") ? JSON.parse(UM.Preferences.getValue("cura/material_settings")) : {}
+    property var materialManagementModel: CuraApplication.getMaterialManagementModel()
 
     property double spoolLength: calculateSpoolLength()
     property real costPerMeter: calculateCostPerMeter()
@@ -565,7 +566,7 @@ TabView
         }
 
         // update the values
-        materialManagement.setMaterialName(base.currentMaterialNode, new_name)
+        base.materialManagementModel.setMaterialName(base.currentMaterialNode, new_name)
         properties.name = new_name
     }