Fix exception when loading processing plugin on Qt 6 builds on KDE

Something internal in KDE itself adds child QObjects to menus, so the
code which tries to find the last action in the menu fails and returns
a QObject instead of a QAction, leading to an exception in the
insertAction call.

This is all too fragile anyway, so avoid it by:

- Just creating the models menu directly in the ui
- Exposing this via interface for use in the processing plugin

Now there's no risk of this code breaking in future if the Project
menu is re-arranged.

python/PyQt6/gui/auto_generated/qgisinterface.sip.in

@@ -245,6 +245,20 @@ Adds an ``action`` to the QGIS "Export project" menu.
 .. seealso:: :py:func:`removeProjectImportAction`
 
 .. versionadded:: 3.30
+%End
+
+    virtual QMenu *projectModelsMenu() = 0;
+%Docstring
+Returns a reference to the main window "Projects" - "Models" submenu.
+
+.. versionadded:: 3.42
+%End
+
+    virtual QMenu *createProjectModelSubMenu( const QString &title ) = 0;
+%Docstring
+Creates a new project model submenu in the "Projects" - "Models" submenu.
+
+.. versionadded:: 3.42
 %End
 
     virtual QMenu *editMenu() = 0;

python/gui/auto_generated/qgisinterface.sip.in

@@ -245,6 +245,20 @@ Adds an ``action`` to the QGIS "Export project" menu.
 .. seealso:: :py:func:`removeProjectImportAction`
 
 .. versionadded:: 3.30
+%End
+
+    virtual QMenu *projectModelsMenu() = 0;
+%Docstring
+Returns a reference to the main window "Projects" - "Models" submenu.
+
+.. versionadded:: 3.42
+%End
+
+    virtual QMenu *createProjectModelSubMenu( const QString &title ) = 0;
+%Docstring
+Creates a new project model submenu in the "Projects" - "Models" submenu.
+
+.. versionadded:: 3.42
 %End
 
     virtual QMenu *editMenu() = 0;

python/plugins/processing/ProcessingPlugin.py

@@ -380,11 +380,6 @@ class ProcessingPlugin(QObject):
         )
         self.sync_in_place_button_state()
 
-        # Sync project models
-        self.projectModelsMenu = None
-        self.projectMenuAction = None
-        self.projectMenuSeparator = None
-
         self.projectProvider = (
             QgsApplication.instance().processingRegistry().providerById("project")
         )
@@ -394,38 +389,32 @@ class ProcessingPlugin(QObject):
 
     def updateProjectModelMenu(self):
         """Add projects models to menu"""
-
-        if self.projectMenuAction is None:
-            self.projectModelsMenu = QMenu(self.tr("Models"))
-            self.projectMenuAction = self.iface.projectMenu().insertMenu(
-                self.iface.projectMenu().children()[-1], self.projectModelsMenu
-            )
-            self.projectMenuAction.setParent(self.projectModelsMenu)
-            self.iface.projectMenu().insertSeparator(self.projectMenuAction)
-
-        self.projectModelsMenu.clear()
+        self.iface.projectModelsMenu().clear()
 
         for model in self.projectProvider.algorithms():
-            modelSubMenu = self.projectModelsMenu.addMenu(model.name())
-            modelSubMenu.setParent(self.projectModelsMenu)
-            action = QAction(self.tr("Execute…"), modelSubMenu)
+            model_sub_menu = self.iface.createProjectModelSubMenu(model.name())
+
+            action = QAction(self.tr("Execute…"))
+            action.setParent(model_sub_menu)
             action.triggered.connect(
                 partial(
                     self.executeAlgorithm,
                     model.id(),
-                    self.projectModelsMenu,
+                    self.iface.projectModelsMenu(),
                     self.toolbox.in_place_mode,
                 )
             )
-            modelSubMenu.addAction(action)
+            model_sub_menu.addAction(action)
             if model.flags() & QgsProcessingAlgorithm.Flag.FlagSupportsBatch:
-                action = QAction(self.tr("Execute as Batch Process…"), modelSubMenu)
-                modelSubMenu.addAction(action)
-                action.triggered.connect(
+                action_batch = QAction(
+                    self.tr("Execute as Batch Process…"), model_sub_menu
+                )
+                model_sub_menu.addAction(action_batch)
+                action_batch.triggered.connect(
                     partial(
                         self.executeAlgorithm,
                         model.id(),
-                        self.projectModelsMenu,
+                        self.iface.projectModelsMenu(),
                         self.toolbox.in_place_mode,
                         True,
                     )
@@ -580,13 +569,6 @@ class ProcessingPlugin(QObject):
         removeButtons()
         removeMenus()
 
-        if self.projectMenuAction is not None:
-            self.iface.projectMenu().removeAction(self.projectMenuAction)
-            self.projectMenuAction = None
-        if self.projectMenuSeparator is not None:
-            self.iface.projectMenu().removeAction(self.projectMenuSeparator)
-            self.projectMenuSeparator = None
-
         QgsGui.historyProviderRegistry().providerById(
             "processing"
         ).executePython.disconnect(self._execute_history_commands)

src/app/qgisapp.h

@@ -665,6 +665,7 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
     //! Menus
     QMenu *projectMenu() { return mProjectMenu; }
     QMenu *projectImportExportMenu() { return menuImport_Export; }
+    QMenu *projectModelsMenu() { return mMenuProjectModels; }
     QMenu *editMenu() { return mEditMenu; }
     QMenu *viewMenu() { return mViewMenu; }
     QMenu *layerMenu() { return mLayerMenu; }

src/app/qgisappinterface.cpp

@@ -733,6 +733,18 @@ void QgisAppInterface::removeProjectExportAction( QAction *action )
   }
 }
 
+QMenu *QgisAppInterface::projectModelsMenu()
+{
+  return qgis->projectModelsMenu();
+}
+
+QMenu *QgisAppInterface::createProjectModelSubMenu( const QString &title )
+{
+  QMenu *modelSubMenu = new QMenu( title, qgis->projectModelsMenu() );
+  qgis->projectModelsMenu()->addMenu( modelSubMenu );
+  return modelSubMenu;
+}
+
 QMenu *QgisAppInterface::editMenu() { return qgis->editMenu(); }
 QMenu *QgisAppInterface::viewMenu() { return qgis->viewMenu(); }
 QMenu *QgisAppInterface::layerMenu() { return qgis->layerMenu(); }

src/app/qgisappinterface.h

@@ -172,6 +172,8 @@ class APP_EXPORT QgisAppInterface : public QgisInterface
     void removeProjectImportAction( QAction *action ) override;
     void addProjectExportAction( QAction *action ) override;
     void removeProjectExportAction( QAction *action ) override;
+    QMenu *projectModelsMenu() override;
+    QMenu *createProjectModelSubMenu( const QString &title ) override;
     QMenu *editMenu() override;
     QMenu *viewMenu() override;
     QMenu *layerMenu() override;

src/gui/qgisinterface.h

@@ -281,6 +281,20 @@ class GUI_EXPORT QgisInterface : public QObject
      */
     virtual void removeProjectExportAction( QAction *action ) = 0;
 
+    /**
+     * Returns a reference to the main window "Projects" - "Models" submenu.
+     *
+     * \since QGIS 3.42
+     */
+    virtual QMenu *projectModelsMenu() = 0;
+
+    /**
+     * Creates a new project model submenu in the "Projects" - "Models" submenu.
+     *
+     * \since QGIS 3.42
+     */
+    virtual QMenu *createProjectModelSubMenu( const QString &title ) = 0;
+
     /**
      * Returns a reference to the main window "Edit" menu.
      */

src/ui/qgisapp.ui

@@ -62,6 +62,11 @@
      <addaction name="separator"/>
      <addaction name="mActionDwgImport"/>
     </widget>
+    <widget class="QMenu" name="mMenuProjectModels">
+     <property name="title">
+      <string>Models</string>
+     </property>
+    </widget>
     <addaction name="mActionNewProject"/>
     <addaction name="mProjectFromTemplateMenu"/>
     <addaction name="separator"/>
@@ -85,6 +90,8 @@
     <addaction name="mActionShowLayoutManager"/>
     <addaction name="mLayoutsMenu"/>
     <addaction name="separator"/>
+    <addaction name="mMenuProjectModels"/>
+    <addaction name="separator"/>
     <addaction name="mActionExit"/>
    </widget>
    <widget class="QMenu" name="mViewMenu">