Fix strange behaviour when app loses access to saf download folder

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() {