added cache folder sweep check before file actions, preventing errors

lib/widget/downloads/download_tray.dart

@@ -7,6 +7,7 @@ import '../../utils/downloads/download_job.dart';
 import '../../utils/downloads/download_manager.dart';
 import '../../utils/haptics.dart';
 import 'downloads_sheet.dart';
+import 'stale_download_guard.dart';
 
 /// Decides whether a just-finished download should open straight in the viewer.
 ///
@@ -198,9 +199,13 @@ class _DownloadTrayHostState extends State<DownloadTrayHost>
 
   void _openJob(DownloadJob job) {
     final path = job.localPath;
-    _manager.markOpened(job);
     final ctx = AppRoutes.overlayContext;
-    if (path == null || ctx == null) return;
+    if (path == null || ctx == null) {
+      _manager.markOpened(job);
+      return;
+    }
+    if (!ensureDownloadStillExists(ctx, job)) return;
+    _manager.markOpened(job);
     Haptics.success();
     AppRoutes.openFileViewer(ctx, path, remoteFile: job.remoteFile);
   }

lib/widget/downloads/downloads_sheet.dart

@@ -10,6 +10,7 @@ import '../../view/pages/files/data/file_type_icon.dart';
 import '../centered_leading.dart';
 import '../details_bottom_sheet.dart';
 import '../info_dialog.dart';
+import 'stale_download_guard.dart';
 
 /// Overview of all active and finished-but-unopened downloads. Lets the user
 /// open/switch between finished files, cancel running ones and retry failures.
@@ -86,8 +87,11 @@ class _DownloadsListState extends State<_DownloadsList> {
   void _open(DownloadJob job) {
     final path = job.localPath;
     if (path == null) return;
-    Haptics.success();
+    // Pop the sheet first so a stale-file dialog lands on the underlying
+    // screen instead of an emptied sheet.
     Navigator.of(widget.sheetContext).pop();
+    if (!ensureDownloadStillExists(widget.rootContext, job)) return;
+    Haptics.success();
     DownloadManager.instance.markOpened(job);
     AppRoutes.openFileViewer(
       widget.rootContext,

lib/widget/downloads/stale_download_guard.dart

@@ -0,0 +1,40 @@
+import 'dart:async';
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+
+import '../../utils/downloads/download_job.dart';
+import '../../utils/downloads/download_manager.dart';
+import '../confirm_dialog.dart';
+import '../info_dialog.dart';
+
+/// Verifies a finished download's cache file is still on disk before opening.
+///
+/// Downloads live in the app's temp dir, which Android may clear at any time
+/// (storage pressure, "Cache leeren") — while completion notifications and
+/// tray entries stay tappable indefinitely. Returns true when the file exists;
+/// otherwise drops the stale job (incl. its notification) and offers a
+/// re-download when the remote path is known (it isn't for notification taps
+/// whose task metadata got lost).
+bool ensureDownloadStillExists(BuildContext context, DownloadJob job) {
+  final path = job.localPath;
+  if (path == null || File(path).existsSync()) return true;
+
+  DownloadManager.instance.markOpened(job);
+  if (job.remotePath.isNotEmpty) {
+    ConfirmDialog(
+      title: 'Datei nicht mehr verfügbar',
+      content:
+          'Die heruntergeladene Datei wurde vom System aus dem Zwischenspeicher entfernt.\nErneut herunterladen?',
+      confirmButton: 'Herunterladen',
+      onConfirm: () => unawaited(DownloadManager.instance.retry(job)),
+    ).asDialog(context);
+  } else {
+    InfoDialog.show(
+      context,
+      'Die heruntergeladene Datei wurde vom System aus dem Zwischenspeicher entfernt. Bitte lade sie erneut herunter.',
+      title: 'Datei nicht mehr verfügbar',
+    );
+  }
+  return false;
+}

lib/widget/file_viewer.dart

@@ -14,8 +14,11 @@ import 'package:share_plus/share_plus.dart';
 import '../routing/app_routes.dart';
 import '../share_intent/remote_file_ref.dart';
 import '../state/app/modules/settings/bloc/settings_cubit.dart';
+import '../utils/downloads/download_manager.dart';
 import 'app_progress_indicator.dart';
+import 'async_action_button.dart';
 import 'centered_leading.dart';
+import 'confirm_dialog.dart';
 import 'file_viewer/code_line.dart';
 import 'file_viewer/deferred_pdf_viewer.dart';
 import 'file_viewer/file_kind.dart';
@@ -82,9 +85,43 @@ class _FileViewerState extends State<FileViewer> {
     super.dispose();
   }
 
+  /// Android may clear the cache dir behind an open viewer at any time —
+  /// verify the file is still there before handing its path to an action.
+  bool _ensureLocalFile() {
+    if (File(widget.path).existsSync()) return true;
+    final remote = widget.remoteFile;
+    if (remote == null) {
+      InfoDialog.show(
+        context,
+        'Die Datei wurde vom System aus dem Zwischenspeicher entfernt. Bitte lade sie erneut herunter.',
+        title: 'Datei nicht mehr verfügbar',
+      );
+      return false;
+    }
+    ConfirmDialog(
+      title: 'Datei nicht mehr verfügbar',
+      content:
+          'Die Datei wurde vom System aus dem Zwischenspeicher entfernt.\nErneut herunterladen?',
+      confirmButton: 'Herunterladen',
+      onConfirm: () {
+        // Pop the viewer before starting so the fresh download auto-opens.
+        Navigator.of(context).pop();
+        unawaited(
+          DownloadManager.instance.start(
+            remotePath: remote.path,
+            name: remote.name,
+            remoteFile: remote,
+          ),
+        );
+      },
+    ).asDialog(context);
+    return false;
+  }
+
   Future<void> _handleAction(FileViewingActions value) async {
     switch (value) {
       case FileViewingActions.openExternal:
+        if (!_ensureLocalFile()) return;
         AppRoutes.openFileViewer(
           context,
           widget.path,
@@ -99,16 +136,21 @@ class _FileViewerState extends State<FileViewer> {
         AppRoutes.openInternalSaveToFolder(context, widget.remoteFile!);
         break;
       case FileViewingActions.share:
+        if (!_ensureLocalFile()) return;
         unawaited(
-          SharePlus.instance.share(
+          runWithErrorDialog(
+            context,
+            () => SharePlus.instance.share(
               ShareParams(
                 files: [XFile(widget.path)],
                 sharePositionOrigin: SharePositionOrigin.get(context),
               ),
             ),
+          ),
         );
         break;
       case FileViewingActions.save:
+        if (!_ensureLocalFile()) return;
         try {
           final source = File(widget.path);
           final size = await source.length();