fix: prevent empty dialog when using bundled distrobox

Two fixes:
1. Add guard in download_distrobox() to prevent duplicate download tasks
   when GSettings watcher synchronously re-triggers the download.
2. Harden TaskManagerDialog to handle already-parented selected_task_view
   by popping/unparenting before pushing a new NavigationPage.

Author: ranfdev

src/dialogs/task_manager_dialog.rs

@@ -125,8 +125,18 @@ mod imp {
             >| {
                 if let Some(task) = task {
                     this.build_task_view(&task);
+                    // If selected_task_view already has a parent from a previous detail page,
+                    // pop the old page and unparent the view to avoid GTK assertion:
+                    // 'gtk_widget_get_parent (child) == NULL'
+                    let stv = &this.imp().selected_task_view;
+                    if stv.parent().is_some() {
+                        this.imp().navigation_view.pop();
+                        if stv.parent().is_some() {
+                            stv.unparent();
+                        }
+                    }
                     this.imp().navigation_view.push(&adw::NavigationPage::new(
-                        &this.imp().selected_task_view,
+                        stv,
                         "Task Details",
                     ));
                 }

src/models/root_store.rs

@@ -346,6 +346,12 @@ impl RootStore {
     }
 
     pub fn download_distrobox(&self) -> DistroboxTask {
+        // Guard: if a download task is already in progress, return it instead of creating a duplicate
+        for task in self.tasks().iter() {
+            if task.name() == "Downloading Distrobox" && !task.ended() {
+                return task;
+            }
+        }
         let task = crate::distrobox_downloader::download_distrobox(self);
         self.tasks().append(&task);
         self.set_selected_task(Some(task.clone()));