Fix strange behaviour when app loses access to saf download folder

app/src/main/java/org/schabi/newpipe/download/DownloadDialog.java

@@ -682,12 +682,15 @@ public class DownloadDialog extends DialogFragment
                 throw new RuntimeException("No stream selected");
         }
 
-        if (!askForSavePath && (mainStorage == null || (mainStorage.isDirect()
-                == NewPipeSettings.useStorageAccessFramework(context)))) {
+        if (!askForSavePath
+                && (mainStorage == null
+                || mainStorage.isDirect() == NewPipeSettings.useStorageAccessFramework(context)
+                || mainStorage.isInvalidSafStorage())) {
             // Pick new download folder if one of:
             // - Download folder is not set
             // - Download folder uses SAF while SAF is disabled
             // - Download folder doesn't use SAF while SAF is enabled
+            // - Download folder uses SAF but the user manually revoked access to it
             Toast.makeText(context, getString(R.string.no_dir_yet),
                     Toast.LENGTH_LONG).show();
 

app/src/main/java/org/schabi/newpipe/streams/io/StoredDirectoryHelper.java

@@ -223,6 +223,15 @@ public class StoredDirectoryHelper {
         return docTree == null ? ioTree.canWrite() : docTree.canWrite();
     }
 
+    /**
+     * @return {@code false} if the storage is direct, or the SAF storage is valid; {@code true} if
+     * SAF access to this SAF storage is denied (e.g. the user clicked on {@code Android settings ->
+     * Apps & notifications -> NewPipe -> Storage & cache -> Clear access});
+     */
+    public boolean isInvalidSafStorage() {
+        return docTree != null && docTree.getName() == null;
+    }
+
     @NonNull
     @Override
     public String toString() {