Fix opening of marketplace button

CURA-8684

resources/qml/MainWindow/ApplicationMenu.qml

@@ -40,7 +40,7 @@ Item
             title: (Qt.platform.os == "osx") ? "&Settings" : catalog.i18nc("@title:menu menubar:toplevel", "&Settings")
         }
 
-        ExtensionMenu {}
+        ExtensionMenu { id: extensionMenu }
 
         PreferencesMenu {}
 
@@ -99,7 +99,7 @@ Item
         target: Cura.Actions.browsePackages
         function onTriggered()
         {
-            curaExtensions.callExtensionMethod("Toolbox", "launch")
+            extensionMenu.extensionModel.callExtensionMethod("Toolbox", "launch")
         }
     }
 
@@ -109,8 +109,8 @@ Item
         target: Cura.Actions.marketplaceMaterials
         function onTriggered()
         {
-            curaExtensions.callExtensionMethod("Toolbox", "launch")
-            curaExtensions.callExtensionMethod("Toolbox", "setViewCategoryToMaterials")
+            extensionMenu.extensionModel.callExtensionMethod("Toolbox", "launch")
+            extensionMenu.extensionModel.callExtensionMethod("Toolbox", "setViewCategoryToMaterials")
         }
     }
 }

resources/qml/Menus/ExtensionMenu.qml

@@ -11,7 +11,7 @@ Menu
 {
     id: extensionMenu
     title: catalog.i18nc("@title:menu menubar:toplevel", "E&xtensions")
-
+    property var extensionModel: UM.ExtensionModel { }
     Component
     {
         id: extensionsMenuItem
@@ -33,7 +33,7 @@ Menu
     Instantiator
     {
         id: extensions
-        model: UM.ExtensionModel { }
+        model: extensionModel
 
         UM.Menu
         {