Enable/Disable Menu items based on plugin loaded

Per @Trinitou suggestions, a plugin specific menu item should only be enabled (available to execute by user)

This fixes the underlying cause of the crashes reported in free-audio#40 and free-audio#42

Therefore this also reverts the now obsolete change in previous merged pull request free-audio#43

Upon load change we update the menu items

Tab indentation set to 3 to be consistent with existing code

Note: Follows the QT memory ownership model, and the QAction pointers lifetime is handled by their parents

Testing:
Launch with no plugin loaded, see menu items are greyed out (disabled), user cannot select these items (as intended) therefore code is not executed and app doesn't crash

Launch with plugin loaded, see menu items are available and executed when user clicks, and confirm still no crashes

Images taken for testing

host/main-window.cc

@@ -46,7 +46,9 @@ void MainWindow::createMenu() {
 
    QMenu *fileMenu = menuBar->addMenu(tr("File"));
    // TODO: fileMenu->addAction(tr("Load plugin"));
-   connect(fileMenu->addAction(tr("Load Native Plugin Preset")),
+
+   _loadPluginPresetAction = fileMenu->addAction(tr("Load Native Plugin Preset"));
+   connect(_loadPluginPresetAction,
            &QAction::triggered,
            this,
            &MainWindow::loadNativePluginPreset);
@@ -62,11 +64,15 @@ void MainWindow::createMenu() {
            &Application::quit);
 
    auto windowsMenu = menuBar->addMenu("Windows");
-   connect(windowsMenu->addAction(tr("Show Parameters")),
+
+   _showPluginParametersAction = windowsMenu->addAction(tr("Show Parameters"));
+   connect(_showPluginParametersAction,
            &QAction::triggered,
            this,
            &MainWindow::showPluginParametersWindow);
-   connect(windowsMenu->addAction(tr("Show Quick Controls")),
+
+   _showPluginQuickControlsAction = windowsMenu->addAction(tr("Show Quick Controls"));
+   connect(_showPluginQuickControlsAction,
            &QAction::triggered,
            this,
            &MainWindow::showPluginQuickControlsWindow);
@@ -76,18 +82,32 @@ void MainWindow::createMenu() {
       dialog.exec();
    });
    menuBar->addSeparator();
-   connect(windowsMenu->addAction(tr("Toggle Plugin Window Visibility")),
+
+   _togglePluginWindowVisibilityAction = windowsMenu->addAction(tr("Toggle Plugin Window Visibility"));
+   connect(_togglePluginWindowVisibilityAction,
            &QAction::triggered,
            this,
            &MainWindow::togglePluginWindowVisibility);
-   connect(windowsMenu->addAction(tr("Recreate Plugin Window")),
+
+   _recreatePluginWindowAction = windowsMenu->addAction(tr("Recreate Plugin Window"));
+   connect(_recreatePluginWindowAction,
            &QAction::triggered,
            this,
            &MainWindow::recreatePluginWindow);
 
    QMenu *helpMenu = menuBar->addMenu(tr("Help"));
    connect(
       helpMenu->addAction(tr("About")), &QAction::triggered, this, &MainWindow::showAboutDialog);
+
+   updateMenuItems();
+}
+
+void MainWindow::updateMenuItems() {
+   _loadPluginPresetAction->setEnabled(_pluginLoaded);
+   _showPluginParametersAction->setEnabled(_pluginLoaded);
+   _showPluginQuickControlsAction->setEnabled(_pluginLoaded);
+   _togglePluginWindowVisibilityAction->setEnabled(_pluginLoaded);
+   _recreatePluginWindowAction->setEnabled(_pluginLoaded);
 }
 
 void MainWindow::showSettingsDialog() {
@@ -117,6 +137,11 @@ void MainWindow::resizePluginView(int width, int height) {
    adjustSize();
 }
 
+void MainWindow::onPluginLoadChange(bool const i_pluginLoaded) {
+   _pluginLoaded = i_pluginLoaded;
+   updateMenuItems();
+}
+
 void MainWindow::loadNativePluginPreset() {
    auto file = QFileDialog::getOpenFileName(this, tr("Load Plugin Native Preset"));
    if (file.isEmpty())

host/main-window.hh

@@ -25,6 +25,7 @@ public:
    void showPluginParametersWindow();
    void showPluginQuickControlsWindow();
    void resizePluginView(int width, int height);
+   void onPluginLoadChange(bool i_pluginLoaded);
 
    void showPluginWindow() { _pluginViewWidget->show(); }
 
@@ -41,11 +42,20 @@ private:
    void togglePluginWindowVisibility();
    void recreatePluginWindow();
    void showAboutDialog();
+   void updateMenuItems();
 
    Application &_application;
    QWindow *_pluginViewWindow = nullptr;
    QWidget *_pluginViewWidget = nullptr;
 
+   QAction *_loadPluginPresetAction = nullptr;
+   QAction *_showPluginParametersAction = nullptr;
+   QAction *_showPluginQuickControlsAction = nullptr;
+   QAction *_togglePluginWindowVisibilityAction = nullptr;
+   QAction *_recreatePluginWindowAction = nullptr;
+
+   bool _pluginLoaded = false;
+
    PluginParametersWidget *_pluginParametersWidget = nullptr;
    PluginQuickControlsWidget *_pluginRemoteControlsWidget = nullptr;
 };

host/plugin-host.cc

@@ -147,12 +147,17 @@ bool PluginHost::load(const QString &path, int pluginIndex) {
 
    scanParams();
    scanQuickControls();
+
+   Application::instance().mainWindow()->onPluginLoadChange(true);
+
    return true;
 }
 
 void PluginHost::unload() {
    checkForMainThread();
 
+   Application::instance().mainWindow()->onPluginLoadChange(false);
+
    if (!_library.isLoaded())
       return;
 
@@ -1171,12 +1176,6 @@ void PluginHost::remoteControlsSuggestPage(clap_id page_id) noexcept {
 bool PluginHost::loadNativePluginPreset(const std::string &path) {
    checkForMainThread();
 
-    if(!_plugin)
-    {
-        std::cerr << "called with a null clap_plugin pointer!" << std::endl;
-        return false;
-    }
-
    if (!_plugin->canUsePresetLoad())
       return false;