Remove usages of deprecated function

resources/qml/Dialogs/WorkspaceSummaryDialog.qml

@@ -140,7 +140,7 @@ UM.Dialog
                     }
                     Label
                     {
-                        text: Cura.MachineManager.activeVariantBuildplateName
+                        text: Cura.activeStack != null ? Cura.MachineManager.activeStack.variant.name : ""
                         width: Math.floor(scroll.width / 3) | 0
                     }
                 }

resources/qml/Menus/ConfigurationMenu/CustomConfiguration.qml

@@ -319,8 +319,8 @@ Item
                 OldControls.ToolButton
                 {
                     id: variantSelection
-                    text: Cura.MachineManager.activeVariantName
-                    tooltip: Cura.MachineManager.activeVariantName
+                    text: Cura.MachineManager.activeStack != null ? Cura.MachineManager.activeStack.variant.name : ""
+                    tooltip: text
                     height: parent.height
                     width: selectors.controlWidth
                     style: UM.Theme.styles.print_setup_header_button

resources/qml/Preferences/Materials/MaterialsPage.qml

@@ -203,7 +203,12 @@ Item
                 var caption = catalog.i18nc("@action:label", "Printer") + ": " + Cura.MachineManager.activeMachineName;
                 if (Cura.MachineManager.hasVariants)
                 {
-                    caption += ", " + Cura.MachineManager.activeDefinitionVariantsName + ": " + Cura.MachineManager.activeVariantName;
+                    var activeVariantName = ""
+                    if(Cura.MachineManager.activeStack != null)
+                    {
+                        activeVariantName =  Cura.MachineManager.activeStack.variant.name
+                    }
+                    caption += ", " + Cura.MachineManager.activeDefinitionVariantsName + ": " + activeVariantName;
                 }
                 return caption;
             }